The germ-plasm; a theory of heredity

Front matter

The Contemporary

Science Series

®t|p i. 1. ItU SItbrarg

Preface

ANY attempt at the present time to work out a theory of heredity in detail may appear to many premature, and ahnost presumptuous : I confess there have been times when it has seemed so even to myself I could not, however, resist the temptation to endeavour to penetrate the mystery of this most marvellous and complex chapter of life as far as my own ability and the present state of our knowledge permitted.

Even though the present attempt may be very imperfect and incomplete, I cannot regard it as premature. Our knowledge has increased during the last twenty years to such an extent, that it does not seem to be altogether a hopeless task to inquire into the actual processes on which the phenomena of heredity depend. It is, moreover, very essential that we should possess a theory of heredity, worked out in such a manner as to suggest new problems, which in their turn will lead to new solutions.

Previous hypotheses have been insufficient in this respect, owing to the fact that they have not been worked out in detail. They are rather to be regarded as paving the way to future theories, by merely formulating explanatory principles without professing to apply them to all the different groups of phenomena which come under the head of heredity, by which means alone their true value can be tested. Even Darwin's theory of * pangenesis' was inade-

ix

X PREFACE

quate in this respect : owing to the comparatively limited number of facts then at his disposal, it could not but be what we may call an ideal theor\- ; that is to say, it is founded upon certain principles without inquiring how far they are based upon actual facts. In themselves, such theories can hardly be looked upon as suggestive, for if once the assumed principle is accepted, all the phenomena are thereby explained, and the matter is open to no further doubt.

Let us assume that the germ contains millions of the primary constituents ('Anlagen') of all the most minute portions of the body ; moreover, that these constituents are always present at the right place and in the right combination during the process of development ; and, further, that they are capable of giving rise in their turn to the parts or organs to which they severally correspond. Such a theory explains everything, or nothing — the premises alone can be attacked. No new problems can arise from it till it has been placed upon a sound basis ; the premises must be shown to be correct, and it must be proved that the germ is actually composed of primary constituents, which by some means or other become combined into groups and are capable of giving rise to the various parts and organs in question. Then, and then only, would the theory serve as an incentive to further investigations into the phenomena of heredity of all kinds, and experiments might be made which would support or contradict it.

There is no doubt a natural tendency to base experiments upon certain preconceived ideas ; but it is one thing to be guided solely by such phenomena as may at the moment appear of especial importance, and another to base operations upon the completed outline of a theory founded upon

PREFACE XI

the principal data bearing upon the question. I have myself more than once abandoned a line of research undertaken in connection with the problem of heredity, because I felt that to proceed without the guidance of a theory more or less complete in itself, and developed on a basis of ascertained facts, would be little better than groping in the dark. The importance of such a theory lies primarily in its suggestiveness, by which alone it becomes a step towards the ideal at which we aim, viz., the formulation of the true and co?nplete theory.

The growth of this book has been very gradual. What first struck me when I began seriously to consider the problem of heredity, some ten years ago, was the necessity for assuming the existence of a special organised and living hereditai-y substance, which in all multicellular organisms, unlike the substance composing the perishable body of the individual, is transmitted from generation to generation. This is the theory of the contimtity of the germ-plasm. My conclusions led me to doubt the usually accepted view of the transmission of va7'iations acqiiired by the body (soma) ; and further research, combined with experiments, tended more and more to strengthen my conviction that in point of fact no such transmission occurs. Meanwhile, the investigations of several distinguished biologists — in which I myself have had some share — on the process of fertilisation and conjugation, brought about a complete revolution in our previous ideas as to the meaning of this process, and further led me to see that the germ-plasm is composed of vital units, each of equal value, but differing in character, containing all the primary constituents of an individual. These ' ancestral germ-plasms ' (' Ahnenplasmen '), or ' ids,' as I now prefer to call them, afforded additional matter where-

Xil PREFACE

with to construct a theory of heredity, though much was wanting to render it complete.

In my last essay I certainly suggested the possibility of solving one of the most difficult problems in heredity — viz., the co-operation of the hereditary substance of the parents in sexual reproduction — by assuming the existence of these ' ids ' ; but I did not for a moment suppose that in doing so I had propounded a complete and elaborated theory of heredity, as some of my readers have thought to be the case ; much still remained to be done first. I had as yet not touched upon such phenomena of heredity as have no direct bearing on the question of sexual reproduction, and had also abstained from any mention of the fundamental point of my theory of heredity — namely, the constitution of the ids. Although I pointed out that they must possess a complex structure which undergoes gradual and regular changes during the development of the individual from the egg-cell, I did not enter into any further details. This question remained in abeyance, for I was by no means sure whether the conception that I had formed on a priori grounds of the minute structure of the ids would prove tenable when viewed in the light of all the many phenomena of heredity. No conclusion could be arrived at respecting the structure of the ids till these phenomena had been individually considered.

All my investigations on the problem of heredity were so far only hnks, to be some day united into a chain which had as yet no existence. The question of the ultimate elements on which to base the theory was the very point on which I remained longest in doubt. The ' pangenesis ' of Darwin, as already mentioned, seemed to me to be far too independent of facts, and even now I am of the opinion that the

PREFACE XUl

very hypothesis from which it derives its name is untenable. There is now scarcely any doubt that the entire conception of the production of the ' gemmules ' by the body-cells, their separation from the latter, and their ' circulation,' is in reality wholly imaginary. In this regard I am still quite as much opposed to Darwin's views as formerly, for I beheve that all parts of the body do not contribute to produce a germ from which the new individual arises, but that, on the contrary, the offspring owes its origin to a peculiar substance of extremely complicated structure, viz., the * germ-plasm.' This substance can never be formed anew ; it can only grow, multiply, and be transmitted from one generation to another. My theory might therefore well be denominated *" bias to -gene sis ' — or origin from a germ-plasm, in contradistinction to Darwin's theory of ^pangenesis ' — or origin from all parts of the body.

My doubts as to the validity of Darwin's theory were for a \, long time not confined to this point alone : the assumption | of the existence oi p7'eformed constituents of all parts of the body seemed to me far too easy a solution of the difficulty, \ besides entailing an impossibility in the shape of an abso- | lutely inconceivable aggregation of primary constituents. I ^ therefore endeavoured to see if it were not possible to imagine that the germ-plasm, though of complex structure, was not composed of such an immense number of particles, and that its further complication arose subsequently in the course of development. In other words, what I sought was a substance from which the whole organism might arise by epigefiesis, and not by evolution* After repeated attempts,

* The theory of * evolution ' or * preformation ' of the early physiologists supposed that all parts of the fully-formed animal or plant were present, in a minute form, in the germ. The rival theory of * epigenesis '

XIV PREFACE

in which I more than once imagined myself successful, but all of which broke down when further tested by facts, I finally became convinced that an epigenetic development is an impossibility. ^Moreover, I found an actual proof of the reality of evolution, which will be explained in the chapter on the structure of the germ-plasm. It is so simple and obvious that. I can scarcely understand how it was possible that it should have escaped my notice so long.

It is gratifying to me to find myself at one with the great English naturalist Darwin, — as well as with de Vries and Wiesner, — at all events in the main point at issue ; and this agreement seems to me to point to the possibility of solving in the end the problem of heredity, which might seem to be open only to the wildest speculations : we may now perhaps hope to succeed in recognising the probable explanations among the vci'diwy possible ones, and in finally selecting from among these the 7ral solution of the problem. This will assuredly be the work of time, and our approach to the truth will be a very gradual one. But our path is marked out ; reasoning supported by observation will lead us to the goal. We are led by the observation of facts to form an opinion as to their bearing on each other. This gives rise to further problems and fresh investigations, which in their turn lead to a new interpretation. In this way light has before now been thrown on many a problem that seemed to bafile explanation. I need only mention the insight that we have now gained into the phenomenon of sexual reproduction.

taught that there is no preformation of parts in the germ, but that the fully-formed organism is produced by a gradual process of differentiation. It will be seen that the word * evolution,' as here used, has no connection with the doctrine of descent with which it is usually connected. — VV. N. E

PREFACE XV

111 the same way we shall succeed in obtaming a firmer and firmer grasp of the problem of heredity, which but a short while ago appeared so utterly unapproachable.

What in this particular question appears to afford additional promise of success is the fact that we can in a sense approach it from two sides ; — namely, by observations, firstly, on the plienomena of heredity, and secondly, on the hereditary substance itself, with which we are now of course acquainted. We can now form an estimate as to whether an explanation of any particular phenomenon of heredity is of a merely hypothetical nature, or whether it may attain to the v^alue of an established fact, inasmuch as we are in a position to judge, within certain limits at all events, whether it is consistent with the actual behaviour of the hereditary substance. Hitherto this has not been possible, and hence all previous theories, including both that of IJarwin's gemmules and of Herbert Spencer's units, were up to a certain point purely speculative. We are now better off in this respect ; and I have no doubt that further research will enable us to penetrate far more deeply still into the complicated processes connected with the idioplasm, if we are prepared to reason on the results of our observations, and to utilise every theoretical advance as an incentive to fresh questions regarding the processes in connection with the distribution of the mysterious nuclear substance.

We are still far from having attained a complete insight into the matter, but I trust nevertheless that the present attempt at a theory of heredity is no mere work of the imagination ; and though it still be no more than an attempt, which will be followed by better ones, I venture to believe that time will prove it to contain more definite points, forming the centre of numerous possibilities, than many will

XVI PREFACE

for the present be prepared to admit. Nevertheless I am well aware that it is but the beginning of a theory, and for this reason I have presented it in the form of an inquiry rather than of an established system. My plan has been not so much to advance doctrines as to propound questions, and to answer them with a greater or lesser degree of certainty, or in some cases even to leave them to be decided by future researches. I do not regard my theory as a complete and perfect one, but trust that it is of such a nature as to be capable of improvement and further development.

It has been my endeavour to write as simply and intelligibly as possible ; not as a specialist writing for speciaHsts, but as one who desires to make his case clear to all interested in biological problems. For this reason a number of figures have been inserted, which, though perhaps superfluous for specialists, will, I trust, assist all who are less conversant with the subject, such as physiologists, medical men, and indeed all interested in natural science, to a clearer conception of the matters under discussion.

As a zoologist, I have naturally, in the first instance, considered the phenomena in their relation to animals, for every one must base his ideas on the facts most famihar to him. I have, however, done my best to lay due weight on the data afforded by the study of plants, and to take into account the views of botanists. It will be seen that the very facts which are furnished by certain hereditary phenomena in plants afford a strong support to certain fundamental points in my theory, and that even those which are at first sight in apparent contradiction, are in reality in perfect accordance with it.

It may perhaps be considered by medical men that I ought to have brought forward more evidence with regard

PREFACE XVI 1

to diseases. We certainly possess a rich material on which observations concerning the transmission of diseases might be based, and this I have made use of so far as seemed expedient. It must, however, not be forgotten that the transmission of so-called hereditary diseases is not always due to a true process of heredity, but in some cases, at any rate, results from an infection of the germ. Unfortunately, we are not always able to distinguish between these two causes ; and as long as this is the case, the data furnished by diseases can only be used with great caution, as will be shown in Chapter XII.

The manuscript of this book was practically completed by the end of April last, but as the translation had then to be made, its publication was delayed for some months. This will account for the fact that no mention, or only a brief one, has been made of researches which have appeared in the interval. My sincerest thanks are due to the translator — Professor W. N. Parker, — whose task has been by no means an easy one : apart from the mere knowledge of the two languages, an intimate acquaintance with the facts treated of and with the whole science of biology is essential in order to render the meaning of this complicated subject clear, and at the same time to reproduce the original text with anything like accuracy. I am of course unable to judge how far Mr. Parker has succeeded in clothing my ideas in good English, but am glad to state that they have been given very correctly, so far as I can judge from those parts which we have discussed together.

In conclusion, I must express my warmest thanks to the Government under which I have the good fortune to live, for the efficient way in which they have seconded my endeavours, by releasing me from my academical duties

XVlll PREFACE

during two winter sessions. My hearty thanks are also due to my friends and colleagues Professors Baumann, Liiroth, Wiedersheim, and Ziegler, as well as to Professor Goebel, of Munich, for information of various kinds ; and I am no less indebted to Miss Else Diestel, who, in addition to much help of a technical nature, has also been at the great pains of preparing an alphabetical index.

I thus venture to bring into the light of day a work which is the fruit of many years labour and of many doubts ; and even though but few of my results should remain unmodified, I hope nevertheless that my work has not been in vain ; for even error, if it originate in correct deductions, must become a step towards truth.

AUGUST WEISMANN.

Freiburg, i/Br., May igth 1892.

Contents

PAGE

A. — HISTORICAL PART i

B. — DESCRIPTIVE PART ... . . .20

Chapter I

1. The fundamental units ....... 37

2. The control of the cell ....... 45

3. The determinants ........ 53

4. The id in ontogeny ........ 60

5. Summary of sections 1-4, relating to the structure of the

germ-plasm . . -75

6. The mechanism for the phyletic changes in the germ-

7. The magnitude of the constituents of the germ-plasm . 85

CHAPTER II Regeneration

1. Its cause and origin in the idoplasm • • • • 93

2. The phylogeny of regeneration . . . . .114

3. Facultative or polygenetic regeneration . . . .126

4. Regeneration in plants . . . . . . .132

5. Regeneration in animal embryos, and the principles of xix

XX CONTENTS

Chapter III Page

Multiplication r,\ Fission —

1. Preliminary remarks ....... 14c

2. The process of fission in the Naidcp . . . .146

3. The process of fission in the i\Iic7-oslomid(T . . . 149

4. The phylogeny of the process of multiplication by fission 151

CHAPTER IV Multiplication by Gemmation

1. The process of gemmation in animals .... 154

a. Coelenterata . . . . . . . .154 c. Tunicata ........ i6o

2. The process of gemmation in plants . . . .163

3. Comparison of the process of gemmation in animals and

plants 166

4. The phylogeny of the process of multiplication by gem-

CHAPTER IX Ontogeny Resulting from the Union of the Germ-Plasm OF TWO Parents

1. The nature of the offspring as determined by the process

of fertilisation ........

2. The share taken by the ancestors in composition of the

germ -plasm .......

3. The struggle of the ids in ontogeny .

a. Plant-hybrids ....

b. Intercalary remarks on variation .

c. The struggle of individual characters

4. The force of heredity ....

5. Summary of Chapter IX.

CHAPTER X The Phenomena of Reversion in their Relation to Amphimixis

1. Reversion to racial characters in plant-hybrids

2. Reversion to individual characters in man

3. Reversion to the characters of ancestors far removed in

animals and plants ....

4. Reversion to rudimentary characters

5. Preliminary summary of sections 1-5

6. Reversion in asexual reproduction .

a. Reversion in the process of gemmation h. Reversion in parthenogenesis

7. Proof that the determinants become disintegrated into

Chapter XIV

1. Normal individual variation . . . . . .410

2. Pathological variation ....... 428

3. Summary of the above two sections and conclusions . 431

4. Variations on a larger scale ...... 435

a. The origin of these variations .... 435

b. The transmission of these variations . . . 444

LIST OF FIGURES

Fig. I. Diagram of nuclear division.

2. Two idants of Ascaris megalocephala.

3. Diagram of the cell-generations in the fore-limb of a newt.

4. Multiplication by fission in a marine worm {A/yriaiiida).

5. Transverse section through the 'zone of gemmation' in a

fresh-water worm (^iVais).

6. An early stage in the formation of a bud in a Hydroid polype

(^Eitdendruini) .

7. Apex of a shoot of a fresh-water Alga (^Chara).

8. Female water-flea {Daphnia) , with summer eggs.

9. Nauplius larva of Leptodora hyalina.

10. Alternation of generations in Botigainvillea.

11. Strobilation of a Medusa.

12. Degeneration of a Medusa into a mere gonophore.

13. Early stages of development of the primitive germ-cells in

Sagitta.

14. Early stages in the development of the primitive germ-cells in

Moina.

15. Early stage of the embryo of Rhabditis nigrovenosa.

16. Diagram of the germ-track in Rhabditis nigrovenosa.

17. Volvox and Pandornia.

18. Process of fertilisation in Ascaris megalocephala .

19. Diagram to illustrate the composition of the idants out of

individually different ids.

20. Formation of spermatozoa in Ascaris viegalocephala.

21. Formation of ova in Ascaris megalocephala.

22. Diagram to illustrate the combination of idants in hybriils.

23. The two varieties of Cypris reptans.

24. Bonellia viridis, male and female.

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H

A. — Historical Part

ERBERT SPENCER was practically the first in the present generation to attempt a theoretical explanation of heredity when he propounded his theory of ' physiological units.' The regeneration of lost parts, e.g., of a leg or the tail of a salamander, led him to the conception of these units, ' in all of which there dwells the intrinsic aptitude to aggregate into the form of that species ; just as in the atoms of a salt there dwells the intrinsic aptitude to crystallise in a particular way.'' He calls this aptitude the ' polarity of the organic units,' and defines the latter as being intermediate between the ' chemical units ' or molecules and the ' morphological units ' or cells. They must be 'immensely more complex than the chemical units,' and must therefore correspond to groups of molecules. It is very interesting at the present day, now that we have advanced somewhat further towards a theory of heredity, to summarise the various aptitudes and forces which Herbert Spencer thought it necessary to ascribe to his ' physiological units,' in order to arrive at an explanation of the phenomena. Although the sections on Heredity and Regeneration constitute only a small portion of his great work on the ' Principles of Biology,' and cannot therefore contain a detailed treatment of the phenomena of heredity, his opinions on this subject are evident.

Spencer considers, on the one hand, that the whole organism is composed of these units, which are all alike in kind, and on the other, that the germ-cells also contain small groups of them. The former supposition makes regeneration possible to each sufficiently large portion of the body, while the latter gives the germ-cell the power of reproducing the whole: inasmuch as the

2 THE (iERM-PI^-VSM

' polarity " of the ' units " leads to their arrangement in such a way that the whole 'crystal' — the organism — is restored, or even formed anew. The mere difference in the arrangement of units alike in kind determines the diversity of \\\^ parts of the body, while the distinction between different species and that between different individitals is due to a diversity in the constitution of the units.

The units of an individual are therefore to a certain extent protean. They are capable of arranging themselves in an immense variety of ways, and so form the most diverse cells, tissues, organs, and parts of the body. But they only do this under the directing influence of the whole, in such a way that the whole forces the units of one part to arrange themselves in just such a way as is necessary for the perfection of that part, — a perfection required for the harmony of the w-hole. Spencer himself says very rightly, ' It seems difficult to conceive that this can be so, but we see that it is so.' As a matter of fact, groups of units removed from an organism possess the power of constructing the whole anew ; and we are thus obliged to admit that the tendency to take a specific form is present in all parts of the organism. The 'units' are physiologically variable quantities, w'hich in every case act in such a manner as the whole demands.

The assumption of these ' physiological units ' does not suffice as an explanation of heredity : it proves insufficient even as interpreting the diflferentiatior). of organs in simple ontogeny, quite apart from the question of amphigonic heredity. But it has the merit of having utilised the smallest vital particles as constituent elements of the organism, and of having made them the basis of a theory of heredity.

Ernst Briicke w-as the first to admit the existence of small vital particles of this kind, and to give cogent reasons for so doing. Although he did not denote them by any special name in his extremely important paper entitled • Elementar Organismen,' * he was the first to oppose the old theory of the cell, especially with regard to its fluid contents, and to show that its body must possess an organisation, quite distinct from the molecular structure of the organic compounds.

Darwin's theory of ' pangenesis ' was stated in the final chapter of his great work on ' The Variation of Animals and Plants

* ' Wiener Sitzungsberichte,' Oct. lo, 1861, Bd. 44, ii., p. 381, under Domestication,' which appeared only a few years after Spencer's ' Principles of Biology.'' The enormous wealth of facts bearing on heredity which is accumulated in this book is in itself sufficient to show how the gifted author felt himself urged on all sides to consider this extremely difficult and complicated problem. For although Darwin modestly described his theory as a provisional hypothesis, his was, nevertheless, the first comprehensive attempt to explain all the known phenomena of heredity by a common principle. The theory has so often been discussed i and is so well known that a brief account of its substance will suffice here.

A multicellular organism, whether animal or vegetable, is gradually built up by cell-division: but it is assumed that this method of multiplication is not the only one. Each cell possesses in addition, at each stage of its development, the power of giving off invisible granules or atoms, which, at a later period and under certain conditions, can develop again into cells similar to those from which they originated. Numbers of these ' gemmules ' are being given off continually from all cells of the body and conveyed into the blood, and thus circulate through the body, finally settling down in some part, principally in those regions in which the development of offspring will take place later on, /.<?., in buds or germ-cells. As gemmules from all the cells of the body are aggregated in these cells, they invest the latter with the power of developing into a new and complete organism. This occurs as follows: — each gemmule reproduces the cell from which it is derived, and the gemmules of the different cells become active in the same order as that in which the corresponding cells followed each other in the ontogeny of the parent.

The germ is not by any means composed exclusively of\ gemmules which have been derived from the organism in which they were formed, but consists, at the same time, of a \er\- large number of gemmules which are derived from parents and ancestors even of very remote generations ; and hence a great many more gemmules take part in each case of ontogeny than there are cells formed. Each cell and each part is represented by a great variety of gemmules. A selection must therefore take place, as only one gemmule can form the required cell, and the rest must remain dormant. In this way a number of gemmules, which have been hitherto dormant, are transferred from

4 THE r.ERM-PIASM

one generation to the next : they may, under certain conditions, l)ecome active, and thus again bring into existence ancestral traits which had disappeared in the parents.

This is, in brief, the theory of pangenesis. It does not take into account the physical nature of the gemmules. They are capable of multiplying, and do so continually : but the question as to whether thev have anv definite arrangement, and if so. what the nature of that arrangement is, is not touched upon ; nor is any mention made of the causes and mechanism bv which it comes about that they are always in the right place and develop into cells at the right time.

I do not say this by any means as a reproach, but only to bring out clearly the speculative character of the whole hypothesis. Darwin did not go on to inquire whether all these assumptions were possible : he only asked what it was necessary to assume in order to explain this or that fact of heredity, without troubling himself to consider whether the assumption were borne out by facts or not. And he was right in doing so, for at the time when he propounded his hypothesis it was not possible to found any theory of heredity on the only sound basis, — that of a knowledge of the most minute cell-structure. I have already pointed out how extremely important and fruitful his theory of pangenesis has been : it drew attention for the first time to all the phenomena that needed explanation, and showed what assumptions must be made in order to explain them.