Incubation and its natural laws

Front matter

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Chapter I. The History Of Artificial Incubation

THE art of artificially hatching hens’ eggs has been known from the remotest ages. Though in Egvpt tradivion attributes the invention to the ancient priests of vue Temple of Isis, it is impossible to determine at what period or to what nation the construction of the first eccaleobion should be credited. The first authentic account we find of the early methods is in ‘‘The Voiage and Travaile of Sir John Maundeville, Kt.,” in his description of the River Nile, Egypt, its geography, productions, etc., written some time previous to 1356, as a “‘solace’”’ during his ‘* wretched rest,” on his return to England from the East, broken in health: * Also at Cayre (Cairo), that I spake of before, sellen men, comounly men and women of other lawe, as we done here bestes in the market. And there is a common hows in that cytee that is all fulle of smale furneys; and thidre bryngen wommen of the toun here eyren (eggs) of hennes, of gees, and of dokes, for to ben put in to the furneyses. And thei that kepen that hows covern hem with hete of hors dong, and outen henne, goos or doke or any other foul; and at the ende of three weeks or a monethe, thei comen agen and taken here chickens and norissche hem and bryngen hem forthe, so that alle the countre isfulle of hem. And so men don there bothe wyntre and somer.” These hatching places at one time extended over Egypt, Damascus, Jerusalem and Samaria. They are now only found in Mausoura, in the village of Berma, situated in the Delta of the Nile. The Egyptians must have had a thorough knowledge of the details of their method, for we find those having ovens, or mamals, and making artificial hatching a business, giving sixty newly hatched chicks in exchange for one hundred eggs.

The stories that reached France of the success of the Egyptians in this branch of rural economy created considerable interest at one time. One Florentine Duke imported an Egyptian director, and met with considerable success. In 1415 Charles VII. became interested, as did also Francis I., at Montrichard, about 1540. Bonneman, a physician of Nauterre, 1777, seems to have been the first to establish hatching ovens whereby the heat was generated with fire and diffused by a circulation of hot water. Though the stories of his establishment are undoubtedly of lusty growth, the fact remains that he had chickens all the year round, and supplied the Imperial Court of France in all seasons. The disastrous events of 1814 were the ruin of his establishment.

To John Champion, Berwick-upon-Tweed, England, 1770, however, probably belongs the credit of first hatching eggs by the aid of fire. He used a room through which passed two heated flues, the eggs being placed on a large, round table in the centre. He claimed that the incubation was performed as well, and as many of the eggs hatched, as if they had been set upon by a hen. He says: ‘The two flue places do not open into the hatching room, but into one adjoining, where the keeper sits and the coal is kept. By this means the eggs are free from smoke and dust, which they might otherwise be greatly injured by. The two rooms have a door communication that the keeper may every now and then visit the eggs, and see if they are in the proper degree of heat.”

From this time on to 1846 little was accomplished, though Bucknell and others attempted the artificial process. James Canteto was more successful, and gave the subject considerable study. The theories that he put forward at that time, though based on no definite data, have been used as a basis for the construction of the various incubators to the present time.

Boston, Mass., were among the pioneers of this line of invention in this country (1870), followed closely by H. J. Haight of New York City, and Messrs. Woodward and Millet .of Charlestown, Mass., in 1871. It is most remarkable that, though the various modifications of the incubator came thick and fast from 1875, none of the inventors have considered it of enough importance to investigate the old and formulate new principles based on reliable data, interpreted by the aid of the known laws of physics, a popular knowledge of which has increased rapidly during the last half century. There has been sufficient time and money expended in perfecting various machines for incubating purposes, but the workers in this line have been groping their way in the dark, and in consequence many of the contrivances which have encumbered the patent list have been based on no principle, justified by no proof, while some, even, are in direct opposition to the laws of nature. They were not familiar with nature’s laws and consequently could not interpret the observed facts of natural incubation, and therefore were not masters of the fundamental principles upon which the success of their invention depended. Upon a knowledge of these laws depends the whole question of right and wrong, of success or failure.

We are confidently told by the enthusiastic poultry litterateur to go to Nature, that she never errs, etc. ’*Tis true that Nature does not err, but human nature is very apt to. We would gain little knowledge of the laws that govern the incubation of an egg from watching a sitting hen for a lifetime, if we have no knowledge of the forces of nature at work, gathered from a close familiarity with the laws that govern analogous phenomena. If we know nothing of gold mining, or of the formations in which gold is found, or the methods for extracting it from the soil, we may dig in rich goldbearing earth for months, and cart it off to fill in sunken lots. ‘e

There is a great difference between that knowledge which is purely intellectual and that which is called natural knowledge. ‘A clever man, shut up alone and allowed unlimited time, might reason out for himself all the truths of mathematics by proceeding from those simple notions of space and number of which he cannot divest himself without ceasing to think, but he could never tell by any effort of reasoning what would become of a lump of sugar if immersed in water, or what impression would be produced on the eye by mixing the colors yellow and blue.” The late Prof. Cooke, in one of his popular lectures, says: ‘‘In practical science the student is taught to follow out a chain of probable evidence, with care and caution to eliminate all accidental phenomena, and supply by experiment and observation the missing links, until he reaches the final conclusion, an intellectual process which, though based wholly on probable evidence, may have all the force and certainty of a mathematical demonstration. Indeed, that highly valued scientific acumen and skill which enables the student to brush away the accidental circumstances by which the laws of nature are always concealed until the truth stands out in bold relief, is but a higher phase of the same talent which marks professional skill in all the higher walks of life. The physician who looks through the external symptoms of his patient to the real disease which lurks beneath; the lawyer who disentangles a mass of conflicting testimony and follows out the truth successfully to the end; the statesman who sees beneath the froth of political life the great fundamental principles which will inevitably rule the conduct of the State, and thus foresees and provides for the coming change ; the general who discovers amid the confusion of the battle-field the weak point of his enemy’s front, and the merchant, even, who can interpret the signs of the unsettled market, employ the same faculty, and in not a much lower degree, that discovered the law of gravitation, and which, since the days of Newton, has worked so successfully to unveil the mysteries of the material creation.” eS

Chapter II. Natural Incubation

In order to understand the requirements of artificial incubation we shall need to first consiler the observed facts in relation to the natural processes, by a comparison of which the general principles common to all may be deduced.

If left to herself ahen will prepare a shallow receptacle for the eggs, to aggregate them in a space that may be covered by the incubating body and keep in the warmth.

When a hen becomes broody after laying a clutch of eggs the plexus of the organ of incubation is developed, which consists of an arterial plexus ramifying beneath the integument of the abdomen, and form, by their unions, a rich network of vessels which becomes truly extraordinary in the time of hatching. Thus the warm blood is brought to the surface which is opposite the eggs, bringing the latter toa temperature that approaches more nearly to that of the blood of the fowl than they would otherwise be brought.

After this preparatory stage a hen sits on the eggs more or less constantly for about twenty days, from time totime moving them about in the nest, probably adjusting them to her own comfort.

As a rule the larger number of a sitting of eggs will become arranged in a circle, with the broad ends forming the circumference. This seems to be due to the hen’s turning or shifting her position on tne eggs, and to the shape of the body of the hen and of the egg itself. During the latter stages of inenbation the broad end becomes very slightly elevatea , which is due in part to the nest becoming more hollow, and to the bulk of the contents of the egg being situated in the small end, making that portion of it the heaviest. é * In settling on the nest the hen makesa slight movement with the body from side to side which partially separates the eggs and forces them out in the feathers, only a comparatively small surface of each egg coming in contact with the incubating body. Over a greater part of that portion of the egg opposite the hen’s body a layer of down and feathers intervenes, which also covers in the outer end and the remaining portion of the surface. Many times the feathers on the abdomen either fall or are plucked off during incubation, and the eggs are then brought into direct contact with the warm body.

The number of times the hen will leave her nest, as well as the period of remaining away from it, varies with | different hens, and the state of the weather. In very warm weather the hen often rises in the nest and throws her feathers out to cool her body, thus partially uncovering the eggs. If the weather be excessively warm during the latter stages of incubation the chicks will often be found dead in the shell.

For a few hours before first breaking the shell the labored breathing of the chicks can be heard, with an occasional pip and chirp, and the eggs sweat. After exclusion the chicks at first sweat profusely, are highly sensitive to the slightest current of air, and huddle close to the hen’s body for warmth.

In tropical climates the heat of the sun is supposed in some instances to be sufficient for incubation, as the ostrich is said to leave her eggs to be hatched by the heat of the sun’s rays alone when she breeds in the neighborhood of the equator, but to sit upon them if inhabiting a more variable climate. Sir Richard Owen says, however, that “‘A steady continuous temperature of about 100 Fahr. is the requisite condition of successful incubation: the heat of the sun alternating with the cold of night would hatch no bird’s egg. The ostrich deposits about fifteen eggs in a hollow of the sand, the male bird incubates, and the young are excluded in from fifty to sixty days.” (American Ostrich, male incubating, 35 days.) ,, ;

The nesting habits of one of our wild ducks, variously known as the Piled-billed Grebe, Little Dipper, or Fish Duck, are of interest, the following notes of which are res

“the little floating island of decaying vegetation held together by mud and moss, which constitutes the nest of this species, is a veritable ornithological curiosity. Imagine a ‘pancake’ of what appears to be mud, measuring twelve or fifteen inches in diameter, and rising two or three inches above the water, which may be from one to three feet in depth; anchor it to the bottom with a few concealed blades of ‘saw-grass,’ in a little open bay, leaving its circumference entirely free ; remove a mass of wet muck from its rounded top and you expose seven oreight soiled brownish-white eggs, resting in a depression the bottom of which is less than an inch from the water: the whole massisconstantly damp. Thisisthe nestof the Dabchick, who is out foraging in the marsh, or perhaps is anxiously watching us from some safe cornernearby. The anchoring blades of coarse saw-grass or flags, being always longer than is necessary to reach the bottom, permit of considerable lateral and vertical movement of the nest, and effectually provide against drowning of the eggs by any ordinary rise in the water-level such as frequently occurs during the prevalence of strong easterly winds on the lake. A small bunch of saw-grass already growing in a suitable situation is evidently selected as a nucleus for the nest, and the tops bent so as to form part of it.

‘* During the day we invariably found the eggs concealed by a covering of muck as above described ; but as we ascertained by repeated visits at night and in the early morning they are uncovered at dusk by the bird who incubates until the morning sun relieves her of her task.”’

The following remarks relating to the breeding hahits of the Black Tern, are also given in the same work : —

““A very common summer resident in the marsh, nesting, orrather laying its eggs on the islands of decaying vegetation and mud formed by sunken muskrat holes. Three eggs constitute a full set, and they are apparently qe SS ak = ; ~~ ak rolled about in the mud until well coated, so as to hide the markings and thereby make them less conspicuous. In two or three instances only did we observe any attempt at a nest, and these would not have been recognized as such without the eggs, consisting as they did of merely a few fragments of grass or bulrushes so disposed as to prevent the eggs from rolling; in most cases the eggs rested in a slight depression on the bare mud. The sun appears to be their chief incubator, although the decaying vegetation, of which the abandoned muskrat houses consist, doubtless plays some part in the process. In no instance did we succeed in flushing a bird from the eggs, although they would appear in pairs to the number of twenty or thirty, and hover about within a few feet of our heads making a great outcry when we approached their property, which was soon to be ours by right of discovery.”

The most striking exception to the rule of incubation is given by the Megapodes or mound-birds, whose range extends from the Phillipines through the islands of the Indian Archipelago and Australasia. A huge mound of decaying vegetable matter is raised, the eggs are deposited vertically in a circle at a certain depth, and the chick is developed with the aid of the heat of fermentation. The egg is of large size, which affords a supply of material sufficing for a unusually advanced state of development of the chick at exclusion, whereby it has strength to force its way to the surface of the hatching mound, with wings and feathers sufficiently developed to enable it to take a flight to the nearest branch of an overshadowing tree.

There are upwards of thirty species of ee family Megapodide, all of which have the peculiarity of burying their eggs in heaps of soil and decaying vegetable substances, trusting to the heat engendered by the decomposing mass for incubation. The mounds differ very much in their composition and form according to location and species of bird. They vary in size from Sfteen feet to 175 feet in circumference, the latter usually being found in the densest thickets, and are doubtless the work of many ages, and of many birds in succession. In the smaller mounds the eggs are deposited so as to form a circle directly in the centre of the mound, at about two feet in depth, separated only by about three inches of earth. In the larger ones but a single egg is laid in each hole, at a depth of from five to seven feet, and after the egg is deposited the earth is immediately thrown down lightly until the hole is filled up.

Chapter III. The Embryonic Development Of The Chick

IT will be found essential to a comprehensive understanding of the Phenomenon of Incubation to have a general knowledge of the Embryonic Development of the Chick.* .

THE STRUCTURE OF THE EGG.

In a fresh egg the following structures are met with:—

The most external is the shell, composed of calcareous matter, the particles of which are deposited in a regular manner, designed to give strength and porosity.

It consists of an outer thinner and an inner thicker layer, both of which are pierced by vertical canals, opening freely on both the exterior and interior surfaces. Through these canals the interchange of gases between the interior and exterior air takes place, and the chemical process of respiration, though feeblé at first, gradually increases in intensity, and is thus carried on during the whole period of incubation.

The appearance to the unaided eye of pores on the surface of the shell is due to the impressions of the villi of the formative membrane, the permeability of the shell by the atmosphere depending on a more minutely porous texture. The first effect of this porosity is the penetration of air between the layers of the lining membrane as

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*I am indebted for the substance of this chapter to Foster & Balfour’s ‘‘Elementg of Embryology,’ to which valuable work the reader is referred for a more minute description than is here given. I aii

ive here the most important facts — those that we shall need to knoé

in discussing the phenomenon of incubation; generalizing where details would only confuse the casual réader, and making such corrections as later observations seem to warrant.

I claim as my own observations the facts relating to the disappearance of the amniotic fluid, and the chick’s then breathing the air contained in the amniotic sac, whereby the pulmonary circulation becomes functionally active before the rupture of the covering membranes.

Figures three and four are from the same work, while one and two are modifications of others. Figures five and six are myown. They give a very good idea of the positions of the chick and other contents of the egg at eleven and sixteen days.

the contents of the egg condense by cold and evaporation after itislaid. The air accumulates at the large end of the egg, between the two membranes, and increases in quantity as the other contents become condensed into the tissues of the chick, and contains a larger per cent. of oxygen than ordinary atmospheric air.

The air space appears as the egg condenses on cooling, and gradually increases in size, whether the egg is incubated or not, as the albumen, which contains 78 per cent. of water in its composition, shrinks in bulk by evapora. tion.

Diagramatic section of an unincubatéd fowl’s egg, bl, blastoderm, or germ-spot; wy, white yolk, consisting of a central flask-shaped mass, and anumber of layers concentrically arranged around it, the outer layer of white yolk lying immediately beneath’the vitelline mem-

sprane, and connected with the central mass beneath the blastoderm:

yy, yellow yolk; v, vitelline membrane; f, layer of more fluid albumen surrounding the yolk; ch, chalaze; a, air chamber between the two layers of the shell membrane; m, shell membranes, where they lie

‘ in contact over the greater portion of the egg; s,shell; d, denser albumen, which extends around the yolk outside-of the internal layer of more fluid albumen; c, boundary between the outer and middle portion of the albumen.

Lining the shell is the shell membrane, which is double, there being an outer thicker and an inner thinner one.

Over the greater part of the egg the two layers of the shell membrane remain permanently in contact (m) ; but at the broad’end they tend to separate, and thus to develop between them a space into which air accumulates (a).

Beneath the shell membrane is the white of the egg, consisting of alternate layers of denser and more fluid albumen, the average composition of which may be taken as 20.40 per cent. proteid matter; 1.60 per cent. saline matter, chiefly sodic and potasic chlorides, with phosphates and sulphates; and 78.00 per cent. water.

Ina hardened egg a spiral arrangement of the white may be observed, and it is possible to tear off thin layers in a spiral direction from left to right from the broad to the narrow end of the egg.

Two twisted cords, termed the Chalaze (ch), composed of coiled membraneous layers of dense albumen,’ run from the two extremities of the egg to the opposite portions of the yolk. Their inner extremities expand and merge into the layer-of denser albumen (d) surrounding the fluid layer (f) next the yolk. The outer extremities are free, and do not quite reach the outer layer of the white. The function of the chalaze is to keep the yolk more steady in the albumen, and to moderate the effects of any violent movement or rotation of the egg. The interior of each chalaze presents the appearance of a succession of opaque white knots.

The yolk is enclosed in the vitelline membrane (v), a transparent and almost structureless membrane, devoid of pores. It is somewhat elastic, and easily thrown into creases and wrinkles.

On the upper surface of the yolk, and immediately under the:vitelline membrane will be seén a little semiopaque spot about one-eighth of an inch in diameter. This is the blastoderm or germ-spot (bl).

The blastoderm of a fertile egg will show an outer white rim, and within that a circular transparent area, in the centre of which, again, there is an opacity, sometimes uniform, and sometimes dotted. In an unfertile egg this white disc is simply marked with a number of irregular clear spaces, there being no proper division into a transparent centre and an opaque rim. This spot in the unfertile egg has sometimes been looked upon as a scar left where the yolk broke from its cell or capsule.

During the descent of the egg along the oviduct of the fowl, where it is exposed to a temperature of 110 degrees or more, the blastoderm undergoes important changes. When the egg is laid and becomes cold, the changes all but entirely cease, and the blastoderm remains inactive until, under the influence of the higher temperature of natural or artificial incubation, the vital activities of the germ are brought back into play, and the arrested changes go on again. The condition of the blastoderm at the time when the egg is laid is not exactly the same in all eggs, though the differences of course are slight. In warm weather changes of the same kind as those caused by actual incubation may take place, to a certain extent, in the interval between laying and incubation.

The blastoderm is always found to be uppermost, whatever may be the position of the egg, provided there is no restraint to the rotation of the yolk. The explanation of this is to be sought for in the lesser specific gravity of that portion of the yolk which is in the neighborhood of the blastoderm. Though to the naked eye the yolk appears tolerably uniform throughout, a section of the yolk of a hardened egg will show that it is not perfectly uniform, but that there is a portion of it having the form of a flask (wy) with a funnel-shaped neck and a number of layers concentrically arranged around it; which, when the egg is boiled, does not become so solid as the rest of the yolk, but remains more or less fiuid. The expanded neck of this flask-shaped space is situated immediately underneath, and where it forms a bed on which the blastoderm (b/) rests; while its bulbous enlargement.is about the middle of tha yolk. This is known as the white yolk (wy), while the great mass of the yolk is composed of what is known as the yellow yolk (yy).

The yolk as a whole is of a lesser specific gravity than the albumen, in which it is immersed, while the pressure of the up-floated germ against the shell-wall is moderated by the weight of the denser albumen forming the chalaze.

The attachments of the chalaze being a little below the axis of the yolk, it adds its weight to the lower half, and thus helps to make the germinal half the lightest and uppermost. Under ordinary circumstances rotation of the egg takes place on its long axis, and if a fresh egg be turned round, the blastoderm will keep its position upwards for one turn or a little more, and then, by the twisting of the chalaze, the yolk is carried completely round, and balances itself again with the blastoderm uppermost in its new position. These modifications of the oviparous egg, so delicately wrought, so wondrously endowed, in anticipatory relation to the needs and conditions of incubation, are scarcely conceivable.

THE BLASTODERMIC EXPANSION,

In the fertile blastoderm, as above noted, the naked eye can distinguish an opaque rim of some breadth surrounding a more transparent central area, in the middle of which again is a white spot of variable appearance. The opaque rim is the commencement of what is termed the area opaca ; the central transparent portion is in the same way the beginning of the area pellucida.

The blastoderm which at first lies with its margin resting on the circular wall of flask-shaped white yolk, spreads out, as a thin circular sheet, over the yolk, immediately under the vitelline membrane. Increasing uniformly at all points of its circumference, the blastodermic expansion covers more and more of the yolk, and at last reaching its opposite pole, completely envelops it. Thus the whole yolk, instead of being enclosed as formerly by the vitelline membrane alone, comes to be also enclosed in a bag formed by the blastoderm, termed the germinal membrane. It is not, however, until quite a late period that the complete closing in at the opposite pole takes place, so that the extension of the blastoderm, or germinal membrane, must be considered as going on during nearly the whole period of incubation. Both the area opaca and the area pellucida share in this enlargement, but the area opaca (Fig. 2, av) increases much more rapidly than the area pellucida (ap), and plays the principal part in encompassing the yolk.

LOOT)

A fowl’s egg after about forty-eight hours incubation, viewed from above, the upper portion of the shell being removed: y, yolk; ao, area opaca, or that portion of the blastoderm which extends over the yolk as the germinal membrane; va, the vascula area of the area opaca, the growth of which is somewhat slower than the germinal membrane; ap, area pellucida, in the centre of which lies the embryo, e.

The circular transparent area, or that part of the area opaca which is nearest the area pellucida, becomes the seat of peculiar changes, which result in the formation of blood-vessels. Hence this part of the area opaca is called the vascula area (va). The embryo (e) itself grows out from the most central portion of the blastoderm, or the central portion of the area pellucida (ap). The head appears first, and several hours later the tail, when the embryo has the appearance of being encased in a small sac and lying in a depression of the yolk-sac, with which it is connected. As incubation proceeds the embrys

20 wd gets larger at the expense of the yolk, which is gradually assimilated by nutritive processes.

Within a day or two of the hatching of the chick, at a time when the yolk-sac is still of some size, or at least has not yet dwindled away altogether, and the development of the embryo is nearly complete, the yolk-sac is slipped into the body of the embryo, so that ultimately the embryo with its coverings alone remain.

The general shape of the embryo 1s due to the direction and slope of the several outgrowths which share in its formation. In addition to this, the tubular sac of the embryo, while everywhere gradually acquiring thicker and thicker walls, undergoes at various points, through local activities of growth in the form of thickenings, ridges, buds or other processes, many modifications of the original outline conferred upon it. Thus, bud-like processes start out from the trunk to form the rudiments of the limbs, and similar thickenings and ridges give rise to the jaws and other parts of the face. By the unequal | development of these outgrowths the body of the chick is gradually moulded into its proper outward shape, At the same time other growths take place within the body walls of the embryo, which result in the formation of the heart, liver, lungs, ete. = ite RS THE AMNION.

Very closely connected with the appearance of the head of the embryo is the formation of the amnion.

The amnion is developed in the following way: There first appears, a little way in front of the head, a fold or thin outgrowth running around in front of it (at). As it increases in height it is gradually drawn backwards over the developing head of the embryo (e). The fold thus covering the head is in due time accompanied by similar folds at some little distance behind the tail, and at some little distance from the sides (B). In this way the embryo bécomes surrounded by a series of folds of thin membrane, which form a continuous wall all around it. All are drawn gradually over the body of the embryo, ee, Omemaser?