Descriptive anatomy : development, microscopic anatomy, bones

Preface

The platform upon which this book was written planned to furnish a clear cut, concise and comprehensive description of Human Anatomy. To this end every sentence has been carefully scrutinized, and all superfluous words eliminated. Similes have been introduced wherever possible, for in spite of their homeliness their mnemonic value is in our opinion undeniable. Conventionalism, therefore, has often been disregarded for the sake of elucidation. The space obtained by condensation of the descriptive text has been devoted to observations on Comparative Anatomy. The consummate perfection of Man is the product of the Mill of Ages, through Phylogenesis and Ontogenesis. Since Darwin has demonstrated our indebtedness to our lowly brethren, the least we can do is to acquaint ourselves with their structure, however humble they are. This perspective adds such interest, instruction, and breadth of view to the study of Human Anatomy that we may be pardoned if the paragraphs on Comparative Anatomy have been drawn at too great length.

The chapter on Development seeks to deal with this important though intricate subject as succinct^ as possible, but frequently the necessity for explanator}^ details could hardly be ignored.

The chapter on Microscopic Anatomy, it is hoped, will fulfil the exactions of Microscopic Pathology, the Ogre that so hideously and mercilessly ravages the beautiful structures formed by it.

The chapter on Bones will, indeed, be sorry reading for him who does not have before the other eye the part of the skeleton undergoing description. Osteology is but interpretation of the hieroglyphical trade-marks indelibly impressed upon Bones by the Mill of Ages.

The type from which this book has been printed has been selected as that least liable to cause eyestrain. This is in keeping with the prophylactic trend of Modern Medicine. We trust that this will compensate for the necessary increase in the number of leaves.

The author wishes to bestow upon Mr. Max J. Walter, of the Medical School, unqualified praise for the efficient manner in which he read the proof-sheets and offered suggestions for their betterment.

241 South Thirteenth Street ,

December 15, igoq.

Chapter 1. Development

The Cell as a Unit.

The cell was established as the unit of tissues by Schwann in 1838. This investigator conceived the cell as a little closed sac that contained protoplasm and a nucleus. Some years later, Virchow announced to the world the doctrine:— “Omnis cellula e cellula,” by which he meant that every cell comes from a pre-existing cell, or, in other words, that no cell arises spontaneously. For example, the ovum (egg), when impregnated by the male spermatic filament, undergoes division into two new cells. These divide into four, the four into eight, and the eight into sixteen, multiplication proceeding until myriads of cells, the aggregate of which constitutes the body, are formed.

On the other hand, the whole organism may be represented by but a single cell, as, for example, the minute animal, the ameba. Though unicellular, yet this animal is capable of performing the functions necessary for its existence, even moving itself about by thrusting forward processes called pseudopods (false feet).

When an organism consists of many cells, these arrange themselves into groups, each performing its allotted function, whether simple or highly specialized. By means of rapid multiplication within a limited area, cells become crowded together and thus assume various shapes, becoming oval or oblong, cubical or polyhedral. Some develop cilia, which expel secretions in the direction of least resistance.

Others become highly specialized, making possible the special senses.

If not compressed, a cell remains round or slightly oval, as instanced by the embryonal mesoblastic cell, and by the blood-cells.

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The Cell as a Whole.

A cell is a nucleated mass of protoplasm. The nucleus is the most important constituent of a cell and is its dynamic force. Nucleated cells are capable of actively functionating and of reproducing others of their kind. A cell that has lost its nucleus, such as the red blood cell, can functionate only passively, in this instance as an oxygen carrier, and cannot reproduce others. It is, moreover, not so longeval as a nucleated cell.

The cell as a whole consists of semifluid protoplasm, which may or may not be enclosed within a cell-wall. That part of the protoplasm exclusive of the nucleus is the cytoplasm. This shows an outer, less highly specialized zone, the exoplasm, and an inner, more highly specialized zone, the endoplasm. That part of the protoplasm allotted to the nucleus is distinguished by the term karyoplasm.

Cytoplasm consists of a network that resembles a cross-section of sponge, appearing honeycombed, and therefore earns the name spongioplasm. Contained within the compartments of this network is a more fluid substance, the hyaloplasm. Cytoplasm may be granular, vesicular, or reticular, according to its appearance under the microscope. It may contain extraneous substances of secretion or of excretion, or present accidentally, and these are included within the term metaplasm.

Karyoplasm, more highly specialized than protoplasm, shows essentially a similar construction. Enclosed within the nuclear membrane, karyoplasm consists of nuclear fibers and nuclear matrix. The nuclear fibers are formed by a network of fine, non-staining threads, the linin, and these threads, with bead-like thickenings at their nodal points, constitute chromatin. The latter, so named in recognition of the strong affinity of its beads for certain dyes, is resolvable chemically into nuclein (i. e. nucleinic acid

MITOSIS in combination with albuminates). Chromatin is of the utmost importance, both because of the active part it assumes in indirect cell-division, and because it is the material basis of the hereditary qualities passed down from one generation to another. The nuclear matrix (nuclear juice) is semifluid, and probably of the same nature as hyaloplasm. It contains paralinin.

The nucleolus is a small, spherical body that rests among the nuclear fibers. It contains a substance called pyrenin. Its function has not been definitely established.

The centrosome is a small body situated in the cytoplasm near the nuclear membrane, often embedded in a little sphere of centroplasm. It may be surrounded by a halo, the centrosphere. It is the dynamic center of the cell.

Cell-Division.

There are two methods by which cells divide — mitotic (indirect) and amitotic (direct). In amitotic division the cell elongates and the nucleus divides so that two nuclei are formed. The entire cell then undergoes constriction in its middle and contracts more and more around the nuclei as they recede from each other, and finally separates into two new cells, each of which contains a nucleus. This process is usually confined to cells whose vitality has been impaired. Mitotic division, on the other hand, is almost the universal rule.

Mitosis.

Mitosis (indirect cell-division) is the cycle of changes a cell undergoes to reproduce two new cells of its kind. Furthermore, through this complicated mechanism is assured equal division of the chromatin of the parent cell, so that each daughter-cell gets exactly one-half the original amount, and also the maintenance of the quota of chromosomes normal to the species.

The number of chromosomes for each cell of a given species is constant. In the human being every somatic cell contains 24 chromosomes. In mitosis, each chromosome splits longitudinally into two, there now being 24 pairs (i. e. 48 in all), one chromosome of each pair going to each daughter-cell, so that each daughter-cell thus receives 24 chromosomes, the constant number. From this it is evident that the chromatin plays the most important part in mitosis. For descriptive purposes, mitosis may be divided into four stages : —

(1) Prophase, in which the cell undergoes changes preparatory to cleavage of the chromosomes.

(2) Metaphase, in which cleavage of the chromosomes and equal division of the chromosomes occur. It is evident that this is the most important stage of the process.

(3) Anaphase, in which the nuclei of the daughter-cells are formed by reapportionment of the chromatin.

(4) Telophase, in which the nuclei pass through the prophase, but in inverse order, and then return to a state of rest until the new cells in their turn undergo mitosis. In this stage the cytoplasm divides, completing the new cells.

Having emphasized the salient features of mitosis, the details of each stage may now be discussed.

Prophase (Stage of Preparation). Although the factors concerned in this stage must be considered seriatim, yet the mind must grasp them as progressing simultaneously. The parts most actively concerned in the prophase are the nuclear constituents and the centrosome, which undergo changes resulting in the mitotic figure. This consists of the chromatin filaments, which stain intensely, and of the achromatic figure, which stains very lightly. The chromatin increases in staining intensity, and from being reticular, arranges itself in a densely convoluted thread, or it may be several threads closely intertwined, forming the close skein. This thickens still more, shortens, and unravels into the loose skein. The latter partitions transversely into a number of V-shaped segments, the chromosomes. During this time the nuclear membrane and nucleolus are disappearing and the chromatin filaments lie free within the karyoplasm, which is now continuous with the cytoplasm.

The centrosome, in the meantime, has divided into two, each of which surrounds itself with a halo, and from each radiate delicate rays, thus forming two asters. These radiate with the centrosomes in a direction tangential to the nucleus

SPERMATIC FILAMENT through arcs of 90 degrees toward opposite poles of the cell, and the rays, prolonged, meet to form an achromatic figure, the amphiaster (double star). This amphiaster is composed of the two asters and the intervening nuclear spindle formed by the prolonged rays.

Metaphase (Stage of Cleavage). The chromosomes undergo longitudinal cleavage. This is the most important stage in mitosis. By it the number of chromosomes is doubled and the amount of chromatin split into halves, each of which goes to each daughter-cell, irrespective of any inequality in size of the latter.

Anaphase (Stage of Separation). The chromosomes arrange themselves about the equator of the achromatic spindle, forming the equatorial plate. Guided by the rays of the achromatic spindle, each of a pair of chromosomes travels toward an opposite pole of the latter, the receding ends of the chromosomes being joined by the connecting filaments.

Telophase (Stage of Reconstruction). The nuclear constituents pass through the stages of the prophase in inverse order. The loose skein is formed, then the close skein, and the nuclear membrane and nucleolus reappear. The protoplasm constricts at right angles to the achromatic spindle and the cell divides. Into each new cell goes half of the nuclear spindle, including an aster and a centrosome. Soon afterward the nuclear spindle vanishes. The two new cells are now complete and pass into a state of rest until they, in turn, undergo mitosis.

It will be again emphatically stated that exactly one-half the amount of chromatin in the mother-cell goes to each daughter-cell, irrespective of any marked inequality in size of the latter.

A syncytium is a multinucleated mass of protoplasm formed in instances of division of the nuclei without cleavage of the protoplasm.

Spermatic Filament.

The spermatic filament is a product of the seminiferous tubules of the testis, and presents a head, a middle-piece, and a tail. The head contains the chromatin. The middle-piece presents at its head-end the end-knob, and here is stored the centrosome. Beginning at the same situation and traversing the middle-piece and tail is the axial filament, which ends beyond the tail as the terminal filament.

Spermatogenesis.

Spermatogenesis is the process by which spermatic filaments are formed from spermatogonia.

Lining the seminiferous tubules of the testis are cells called spermatogonia. Each spermatogone divides into two cells, one of which takes the place the spermatogone occupied before division, whilst the other becomes the primary spermatocyte. By dividing this generates two secondary spermatocytes. These divide and produce four spermatids. Without any further division, but by a complicated process of histogenesis, the four spermatids mature into four spermatic filaments. Thus, from each primary spermatocyte, by two successive divisions, four equivalent spermatids are formed, and each of these directly becomes a spermatic filament. This process is described more in detail with the male generative organs. It has been estimated that 340,000,000,000 spermatic filaments are produced in a lifetime.

Ovum.

The ovum (egg) represents the most perfect type of cell. It is formed in the ovary which, it has been computed, contains about 30,000 at birth. Of these many degenerate, others never mature, and only an infinitesimal number become fertilized. Only about 200 become mature.

The human ovum is about 0.2 mm. in diameter, and is just visible macroscopically as a clear speck. It is surrounded by the zona radiata, which is a protecting membrane and not the true cell-wall. Outside of this are the layers of the vesicular follicle, which will be described with the female generative organs.

The ovum contains the essential features of a typical cell, but to these special names are given. It often has a

MATURATION OF OVUM distinct cell-wall, the vitelline membrane. This encloses the vitellus (yolk), which consists of ooplasm (vice cytoplasm), and of nutritive granules, the deutoplasm.

The germinal vesicle (nucleus) is enclosed within a nuclear membrane, and the germinal spot (nucleolus) lies within it. The centrosome occupies its usual position just without the nuclear membrane, but is not always visible.

Classification of Ova.

Ova are classified according to the amount and distribution of the deutoplasm thus: —

(1) Homolecithal, in which the deutoplasm is meager in amount and uniformly distributed throughout the vitellus. Such is the mammalian ovum.

(2) Telolecithal, in which the ooplasm is accumulated at one pole and the deutoplasm at the other. The ooplasm is the formative yolk and occupies the animal pole, while the deutoplasm is the nutritive yolk and occupies the vegetative pole. Such are the eggs of Birds, Reptiles, Fishes, and Amphibians.

(3) Centrolecithal, in which the ooplasm is confined to the periphery and the deutoplasm to the center. Such are the eggs of some Insects.

Maturation of Ovum.

Maturation signifies the changes that prepare the ovum for fertilization. Before it can functionate, every immature ovum must undergo this process. It will be seen that the object of maturation is to reduce the number of chromosomes by one-half, so that the matured ovum contains one-half the number of chromosomes a somatic cell possesses. The same result is accomplished in spermatogenesis, the spermatic filament having half as many chromosomes as its ancestor, the spermatogone. It is understood, of course, that the primordial sexual cells and the somatic cells have an equal number of chromosomes. This number is constant in every species, and in man is now regarded as twenty-four. If, then, the number of chromosomes in a spermatic filament and in a matured ovum are one-half those of a somatic or primordial sexual cell, it follows that a spermatic filament and a matured ovum each possesses twelve chromosomes. Why is such a reduction necessary? If, during fertilization, the sperm-nucleus and the egg-nucleus contributed 24 chromosomes apiece to the segmentation-nucleus, the latter would contain 48 chromosomes, and each of the two blastomeres would have the same number, consequent upon mitosis of the segmentation-nucleus. And every cell in the new individual would have twice the number of chromosomes constant for man. Fertilization will be described more fully presently.

Before birth, the ovary contains primordial ova, which, in the course of their growth and division, give rise to oogonia. From the oogonia are developed primary oocytes, and by birth all the oogonia have terminated in primary oocytes. These initiate a cycle of two divisions in the course of which reduction of chromosomes occurs, and at the end of which four ova are produced. Of these, one is full and blooming and three are abortive.

The primary oocyte divides by mitosis into two secondary oocytes. While both receive an equal apportionment of chromatin, one receives nearly all the cytoplasm; the other, therefore, is a small cell, the first polar body. The secondary oocyte repeats the mitosis, giving rise to the matured ovum and the second polar body. The similarity between maturation and spermatogenesis is completed when the first polar body divides, thus making three polar bodies. These, with the matured ovum, correspond to the four spermatids. The three polar bodies have been referred to as abortive ova. They seem to sacrifice themselves so that the fourth division-product, the matured ovum, may gain additional nutritive material and cytoplasm for more robust development. The nucleus of the matured ovum is called the egg-nucleus.

At what stage of maturation, and by what mechanism is reduction of the chromosomes brought about? The oogonia contain 24 chromosomes and the secondary oocytes 12 each. Therefore, the reduction must occur in the primary oocytes. The mechanism by which reduction is brought about is termed heterotype mitosis, to comprehend which a

MATURATION OF OVUM thorough understanding of the ordinary homotype mitosis is absolutely essential, and it would be well to review the latter, described a few pages back.

In the prophase of heterotype mitosis the close skein of chromatin withdraws to one side of the nucleus, a condition termed synapsis. During this synapsis the 24 chromosomes fuse so as to form 12 pairs. In the metaphase, instead of cleaving longitudinally as in homotype mitosis, the two parts of each pair separate, one part of each pair attaching itself to the threads of the achromatic spindle, by which it is drawn to one secondary oocyte, whilst the other part is drawn in like manner to the other secondary oocyte. Each part consequently represents a whole chromosome. Therefore, from the 12 fused pairs, 24 single chromosomes are distributed to both secondary oocytes-— 12 to each.

While the drawing of 12 whole chromosomes along the threads of each half of the achromatic spindle to each secondary oocyte seems the natural course for the completion of reduction, and while it would simplify matters were it so, yet a further complication of heterotpye mitosis must be described to complete this subject. It would be permissible, even preferable, however, to conclude with the above interpretation rather than misconceive the following course oi events.

As the 12 whole chromosomes travel along the threads of each half of the achromatic spindle toward each secondary oocyte, they undergo cleavage so that each secondary oocyte receives 24 half-chromosomes instead of 12 whole ones. Even so, the result is the same, since the 24 half-chromosomes are distributed by their secondary oocyte to the matured ovum (or second polar body) directly without further cleavage, so that finally either of the two cells mentioned gets 12 of the 24 half-chromosomes. In other words, the longitudinal cleavage that should have occurred in the metaphase of the secondary oocyte actually takes place in the 12 whole chromosomes during their transit between the equator of the achromatic spindle of the primary oocyte and the nucleus of the respective secondary oocyte. Therefore, there can be no complete reconstruction of the nucleus of a secondary oocyte.

Comparing maturation with spermatogenesis, it is seen that in the former process the third generation yields the matured ovum, whilst in the latter process the third generation yields the immature spermatid. The matured ovum is thus the equivalent, ontogenetically, of the spermatid directly, and of the spermatic filament indirectly, since the latter is formed from the spermatid by a complicated process of histogenesis without, however, any alteration of the status of generation or of the number of chromosomes. Furthermore, in the comparison, while all four products of spermatogenesis become full-fledged spermatic filaments, only one of the four products of oogensis becomes a functional ovum.

An exceptional amount of space has been devoted to the description of maturation not only because of its fundamental importance, but also because it has always proven itself a perplexing problem to grasp.

Fertilization.

Heretofore the spermatic filament and ovum have been considered as individual elements. In fertilization thev

wf

unify and, losing their identity, give rise to two new cells from which a new being is ultimately formed.

In Hydra, certain Worms, and other lowly forms reproduction may occur by division of the whole organism into two parts, or by separation of a portion of the body from the parent organism. This illustrates non-sexual reproduction. In others, as Phylloxera, Bees, and Water-fleas, the germcells are capable of developing without having been fertilized, by the process of parthogenesis. In higher forms, however, fertilization is necessary for reproduction.

Maturation takes place while the vesicular follicle is enlarging and approaching the surface of the ovary. Arrived here, the vesicular follicle bursts, and the ovum, now matured, is discharged in the process of ovulation. When this is about to occur, the ovary is grasped by the fimbriated extremity of the oviduct, the cilia of which propel the received ovum into the lumen of the oviduct. The outer third of the latter is usually the trysting-place for the ovum and the spermatic filament.

The seminal fluid, deposited in the vaginal fornices,

FERTILIZATION bathes the cervix of the uterus, to whose cavity it affords the spermatic filaments entrance. Their forward march is resisted by the cervical plug of mucus, by the contrary currents of the cilia lining the uterus and oviduct, and by the labyrinthine path within the latter. The spermatic filaments, however, propelled by their tails at the rate of 2.5 mm. per minute, cover the 10 inches between the upper part of the vagina and the outer third of the oviduct in about 9 hours. Were it not for the obstacles in their path, they could reduce the time to an hour and a half. The spermatic filament and the ovum retain their vitality for about two weeks, so that if they meet within this time, fertilization is possible.

It has been estimated that every cubic millimeter of seminal fluid contains about 60,000 spermatic filaments, and that the total number deposited with each ejaculation is near 200,000,000. Of these, only a few reach their goal and only one, the winner of the obstacle-race, finally impregnates the ovum, the rest being barred by a dense membrane that, like a torpedo-net, is thrown out around the ovum directly upon the entrance of one filament. In the Newt, however, polyspermy is the rule, but of the many spermatic filaments that penetrate the ovum, only one unites with the egg-nucleus, the rest being absorbed by the cytoplasm of the ovum.

Immediately the spermatic filament impinges upon the ovum, the latter sends from its periphery the receptive eminence, into which the head and middle piece penetrate, the tail, no longer of service, being cast off. In Physa fontinalis, and in the Bat, however, the whole spermatic filament, as a rule, enters the ovum. The middle-piece becomes indistinguishable by radiations that appear and the head, now the sperm-nucleus, moves toward the center of the ovum. The sperm-nucleus and the egg-nucleus now lie side-by-side. In each the nuclear membrane disappears and the chromatin is arranged as the close skein, then the loose skein, and finally as chromosomes. The 12 chromosomes of each thus form separate groups arranged beside each other, the chromosomes not mixing until the segmentation-nucleus has divided. There is now formed an achromatic spindle, the asters of which are supplied by the centrosome either of the spermatic filament or of the ovum, or else from one arising de novo, according to different views. About the equatorial plate of this achromatic spindle assemble the chromosomes — the 12 paternal arrayed on one side, and the 12 maternal on the other. This figure constitutes the segmentation-nucleus. In the metaphase each group of chromosomes undergoes longitudinal cleavage, so that there are 24 paternal half-chromosomes, and 24 maternal half-chromosomes. The fertilized ovum now divides into two cells, the primary blastomeres, each of which receives 12 paternal half-chromosomes and 12 maternal half-chromosomes, so that each finally contains 24 chromosomes, the number constant for the human species. Thus, the purposes of maturation and of spermatogenesis are fulfilled. And in all subsequent mitoses 24 chromosomes are found.

Segmentation.

Segmentation is the process inaugurated by the division of the fertilized ovum into two new cells, and eventuates ultimately in the formation of a new being. The two cells, termed blastomeres, divide into four, the four into eight, and the eight into sixteen, this binary division continuing until the tendency to subdivision is temporarily arrested. There is thus formed a mass of cells termed the morula, which is enclosed within the zona radiata, and which is about the same size as the ovum that it has replaced. The morula is a solid, compact, spherical mass of cells that resembles a mulberry. The number of cells it contains is unknown, but the number of cells ultimately derived from it (i. e., in an adult man) has been computed at 26,500,000,000,000.

i •

Types of Segmentation.

T. Complete (Holoblastic

• ova).

• ha) Equal (Amphioxus and 1 mammals, except Mono-

tremes).

- b) Unequal (Amphibit'.-T ; ans, Cyclostomes).

II. Partial (Meroblastic ova).

a) Discoidal (Birds, Rep tiles, Monotremes).

b) Superficial (Insects),

FORMATION OF GERM LAYERS

A holoblastic ovum is one in which the entire ovum with all the yolk is included in the process of segmentation. When equal, it occurs in homolecithal ova. The human egg, therefore, is a homolecithal holoblastic ovum. The eggs of the Amphioxus present the simplest and purest type of complete equal segmentation. When unequal, it occurs in moderately telolecithal ova. Here the formative cells at the animal pole multiply more rapidly than the cells at the vegetative pole, the latter being retarded by the accumulation of the yolk. In this form the animal pole is above and the vegetative pole below, this position being due to the heavier weight of the deutoplasm over the ooplasm.

A meroblastic ovum is one in which only part of the ovum is included in the process of segmentation. Partial discoidal segmentation occurs in telolecithal ova in which the vegetative pole is bulky. The inertia of this deutoplasm retards division at the vegetative pole, so that it is only within the limited extent of the animal pole that active proliferation occurs. Partial superficial segmentation occurs in centrolecithal ova, in which the peripheral ooplasm alone segments.

Formation of Germ Layers.

Certain cells of the morula divide more rapidly, take a superficial position, and come to cover and enclose the remainder. There is thus early distinguished a peripheral layer and a central mass. Some of the cells in the latter gradually vacuolate, forming within the morula a cavity termed the blastocele, which is occupied by prolymph.

This vacuolization represents a belated formation of yolk, and the blastocele is equivalent to the yolk-mass of an ovum such as the reptilian. The blastocele gradually increases in size until the morula, from being a solid sphere, becomes a vesicle whose wall, the peripheral layer, is composed of but a single thickness of cells. The morula, thus modified, has become the blastocyst (blastodermic vesicle).

The blastocyst is composed of trophoblast and an inner cell-mass. The trophoblast represents the single peripheral layer of cells that forms its wall. The trophoblast (nourishing layer), while taking no part in the building up of the em - bryo, yet forms a bond of union between the latter and the maternal tissues, thus nourishing the embryo, as its name impli es. The inner cell-mass is that part of the central mass of the morula that has been spared liquefaction. Attached to the inner surface of the trophoblast at one spot, the embryonic spot, it projects as a rounded knob of cells into the cavity of the blastocyst. From it are eventually derived the embryo, yolk-sac, and amnion. The blastocyst is l-120th. of an inch in diameter.

Within a short space of time the blastocyst increases to l-25th. of an inch in diameter, so quickly that the trophoblast is flattened out from rapid stretching. The trophoblastic cells directly overlying the inner cell-mass are termed cells of Rauber. From the pressure consequent upon the rapid proliferation of its cells, as well as from the great expansion of the trophoblast, the inner cell-mass erupts through the overlying cells of Rauber, causing them to disappear. The inner cell-mass in this way reaches the surface and its margins, which correspond with those of the embryonic spot formed by it, are continuous with the trophoblast. The wall of the blastocyst at this stage, then, is formed by trophoblast except at the embryonic spot, where it is formed by the inner cell-mass.

From the part of the inner cell-mass next the cavity of the blastocyst, a layer of cells is split off to form the entoblast. (The suffix “blast” is a designation for a formative cell, and is used in many combinations with a prefix denoting the structure which it produces). This forms a small, closed sac, the vitelline sac, which is separated from the trophoblastic wall by a space, made larger by the rapid expansion of the latter.

The remainder of the inner cell-mass, which is left after splitting off of the entoblast, is termed ectoblast. Vacuolation of the core of the ectoblast gives rise to the amniotic cavity. The ectoblast roofing in this cavity is termed amniotic ectoblast, whilst that forming its floor is the embryonic ectoblast, since it participates in the formation of the embryo. This is the stage of the bilaminar blastocyst. In Coelenterata, differentiation does not pass beyond this stage.

A third layer, the mesoblast, soon appears and insinu-

FORMATION OF GERM LAYERS ates itself between the ectoblast and entoblast in a manner to be described presently. This is the stage of the trilaminar blastocyst. Concerning the origin of the mesoblast, no definite statement can be made. According to some it arises from the entoblast, whilst others dispose of it as having an* obscure origin near the primitive streak (q. v.).

From the layers of the trilaminar blastocyst are derived all the tissues of the body. The cells of the ectoblast and entoblast are characterized by being closely packed together, and by having a meagre amount of intercellular substance. The cells of the mesoblast, on the other hand, are more loosely arranged, and the amount of intercellular substance is large. Each layer has a definite physiological task to fulfil. The ectoblast is protective, clothing the body as the epidermis of the skin. The mesoblast is supportive, forming the great mass of connective tissue. The entoblast is nutritive, forming the gastropulmonary system. These statements are very general when it is considered that from the ectoblast is also derived the entire nervous system; from the mesoblast, such important systems as the muscular, circulatory, and urinary; and from the entoblast, the liver and pancreas.