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Foster M. and Sedgwick A. The Works of Francis Balfour Vol. II. A Treatise on Comparative Embryology 1. (1885) MacMillan and Co., London.
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. I. Separate Memoirs (1885) MacMillan and Co., London.
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. II. A Treatise on Comparative Embryology 1. (1885) MacMillan and Co., London.
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London.
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. IV. Plates (1885) MacMillan and Co., London.
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The Works of Francis Balfour
Volume II. A Treatise on Comparative Embryology 1
Francis Maitland Balfour, M.A., Ll.D., F.R.S.,
Fellow Of Trinity College, And Professor Of Animal Morphology In The University Of Cambridge.
Edited By M. Foster, F.R.S.,
Professor Of Physiology In The University Of Cambridge; And
Adam Sedgwick, M.A.,
Fellow And Lecturer Of Trinity College, Cambridge.
Vol. I. Inventebrata.
MACMILLAN AND CO. 1885
My aim in writing this work has been to give such an account of the development of animal forms as may prove useful both to students and to those engaged in embryological research. The present volume, save in the introductory chapters, is limited to a description of the development of the Invertebrata: the second and concluding volume will deal with the Vertebrata, and with the special histories of the several organs.
Since the work is, I believe, with the exception of a small but useful volume by Packard, the first attempt to deal in a complete manner with the whole science of Embryology in its recent aspects, and since a large portion of the matter contained in it is not to be found in the ordinary text books, it appeared desirable to give unusually ample references to original sources. I have accordingly placed at the end of each chapter, or in some cases of each section of a chapter, a list of the more important papers referring to the subject dealt with. The papers in each list are numbered continuously, and are referred to in the text by their numbers. These lists are reprinted as an appendix at the end of each volume. It will of course be understood that they do not profess to form a complete bibliography of the subject.
In order to facilitate the use of the work by students I have employed two types. The more general parts of the work are printed in large type; while a smaller type is used for much of the theoretical matter, for the details of various special modes of development, for the histories of the less important forms, and for controversial matter generally. The student, especially when commencing his studies in Embryology, may advantageously confine his attention to the matter in the larger type; it is of course assumed that he already possesses a competent knowledge of Comparative Anatomy.
Since the theory of evolution became accepted as an established doctrine, the important bearings of Embryol&y on a morphological views have been universally recognised; but the very vigour with which this department of science has been pursued during the last few years has led to the appearance of a large number of incomplete and contradictory observations and theories; and to arrange these into anything like an orderly and systematic exposition has been no easy task. Many Embryologists will indeed probably hold that any attempt to do so at the present time is premature, and therefore doomed to failure. I must leave it to others to decide how far my effort has been justified. That what I have written contains errors and shortcomings is I fear only too certain, but I trust that those who are most capable of detecting them will also be most charitable in excusing them.
The work is fully illustrated, and most of the figures have been especially engraved from original memoirs or from my own papers or drawings by Mr Collings, who has spared no pains to r ndcr the woodcuts as clear and intelligible as possible. I trust my readers will not be disappointed with the results. The sources from which the woodcuts are taken have been in all cases acknowledged, and in the. cases where no source is given the illustrations are my own.
I take this opportunity of acknowledging my great obligations to Professors Agassiz, Huxley, Gegenbaur, Lankester, Turner, Kolliker, and Claus, to Sir John Lubbock, Mr Moseley, and Mr P. H. Carpenter, for the use of electrotypes of woodcuts from their works.
I am also under great obligations to numerous friends who have helped me in various ways in the course of my labour. Professor Kleinenberg, of Messina, has read through the whole of the proofs, and has made numerous valuable criticisms. My friend and former pupil, Mr Adam Sedgwick, has been of the greatest assistance to me in correcting the proofs. I have had the benefit of many useful suggestions by Professor Lankester especially in the chapter on the Mollusca, and Mr P. H. Carpenter has kindly revised the chapter on the Echinodermata.
I am also much indebted to Dr Michael Foster, Mr Moseley, and Mr Dew-Smith for aid and advice.
Contents of Volume I.
- The Ovum and Spermatozoon General history of the Ovum, Special history of the Ovum in different types, The Spermatozoon.
- The Maturation and Impregnation of the Ovum Maturation of the Ovum, and formation of the polar bodies, Impregnation of the Ovum, Summary.
- The Segmentation of the OvumInternal phenomena of Segmentation, External features of Segmentation.
- Dicyemae and Orthonectidae Dicyema
- Coelenterata Hydrozoa, Actinozoa, Ctenophora, Summary, etc., Alternations of generations.
- Platyhelminthes Turbellaria, Nemertea, Trematoda, Cestoda.
- Mollusca Formation of the layers and larval characters, Gasteropoda and Pteropoda, Cephalopo<ia, Polyplacophora, Scaphopoda, Lamellibramhiata, General review of Mollnscan Larva, Development of organs.
- Polyzoa Entoprocta, Ectoprocta, Summary and general considerations.
- Brachiopoda Development of the layers, The history of the larva, Development of organs, General observations on the affinities of the Brachiopoda.
- Chilopoda Formation of the germinal layers, The larval form, Formation of organs, Alternations of generations.
- Discophora Formation of layers, History of larva.
- Gephyrea Gephyrea nuda, Gephyrea tubicola, General considerations.
- Chaetognatha Myzostomea, Gastrotricha.
- Nemathelminthes Acanthocephala,
- Tracheata. I'rototrachcata, Myriapoda, Insecta, Embryonic mcmbrants and the formation of the layers, Formation of the organs, Special types of larvae, Mctamorfikoiis and heterogamy, Arachnida, Formation of the layer and general development, Formation of the organs, Formation of the layers and embryonic envelopes in the Tracheata.
- Crustacea History of larval forms. Branchiopoda, Malacostraca, Copepoda, Cirripedia, Ostracoda, Phylogeny of the Crustacea, The formation of the germinal layers, Comparative development of organs.
- CHAPTER XIX. Pcecilopoda Pycnogonida, Pentastomida, Tardigrada, Summary of Arthropodan Development.
- Echinodermata Development of the germinal layers, Development of the larval appendages and metamorphosis, Summary and general considerations.
Embryology forms a large and important department of Biology. Strictly interpreted according to the meaning of the word, it ought to deal with the growth and structure of organisms during their development within the egg membranes, before they are capable of leading an independent existence. Modern investigations have however shewn that such a limitation of the science would have a purely artificial character, and the term Embryology is now employed to cover the anatomy and physiology of the organism during the whole period included between its first coming into being and its attainment of the adult state.
The subject-matter of the science of Embryology admits of a twofold classification. It may be placed under a series of heads, each dealing either with a special group of organisms, or with a special department of the whole science. If classified in the first of these ways the science will naturally be divided into an Embryology of Plants, and an Embryology of Animals ; each of which admits of further subdivision. In the second way the subject falls under two primary heads ; viz. Physiological Embryology and Anatomical Embryology.
The present treatise deals only with the Embryology of Animals, and is further confined to those animals known as Metazoa. The science is moreover treated from the morphological or anatomical, rather than from the physiological side.
The marvellous phenomenon of the evolution of a highly complicated living being from a simple undifferentiated germ in which it needs the aid of the most modern microscopical appliances to detect any visible signs of life, has not unnaturally attracted the attention of biologists from the very earliest periods. Before the establishment of the cell theory the origin of the organism from the germ was not known to be an occurrence of the same nature as the growth of the fully formed individual, and Embryological investigations were mixed up with irrelevant speculations on the origin of life 1 .
The difficulties of understanding the formation of the individual from the structureless germ led anatomists at one time to accept the view "according to which the embryo preexisted, "even though invisible, in the ovum, and the changes which "took place during incubation consisted not in a formation of " parts, but in a growth, i.e. in an expansion with concomitant " changes of the already existing germ."
Great as is the interest attaching to the simple and isolated life histories of individual organisms, this interest has been increased tenfold by the generalizations of Mr Charles Darwin.
It has long been recognized that the embryos and larvae of the higher forms of each group pass, in the course of their development, through a series of stages in which they more or less completely resemble the lower forms of the group 2 . This remarkable phenomenon receives its explanation on Mr Darwin's theory of descent. There are, according to this theory, two guiding, and in a certain sense antagonistic principles which have rendered possible the present order of the organic world. These are known as the laws of heredity and variation. The first of these laws asserts that the characters of an organism at all stages of its existence are reproduced in its descendants at corresponding stages. The second of these laws asserts that offspring never exactly resemble their parents. By the common action of these two principles continuous variation from a parent type becomes a possibility, since every acquired variation has a tendency to be inherited.
1 To this general statement Wolff forms a remarkable exception, for though without any clear knowledge of what we call cells he had very distinct notions on the relations of growth and development.
Von Boer who is often stated to have established the above generalization really maintained a somewhat different view. He held (Ueber Entwickelungsgeschichte d. Tkiere, p. 314) that the embryos of higher forms never resembled the adult stages of lower forms but merely the embryos of such forms. Von Bacr was mistaken in thus absolutely limiting the generalization, but his statement is much more nearly true than a definite statement of the exact similarity of the embryos of higher forms to the adults of lower ones.
The remarkable law of development enunciated above, which has been extended, especially by the researches of Huxley 1 and Kowalevsky, beyond the limits of the more or less artificial groups created by naturalists, to the whole animal kingdom, is a special case of the law of heredity. This law, interpreted in accordance with the theory of descent, asserts that each organism in the course of its individual ontogeny repeats the history of its ancestral development. It may be stated in another way so as to bring out its intimate connection with the laws of inheritance and variation. Each organism reproduces the variations inherited from all its ancestors at successive stages in its individual ontogeny which correspond with those at which the variations appeared in its ancestors. This mode of stating the law shews that it is a necessary consequence of the law of inheritance. The above considerations clearly bring out the fact that Comparative Embryology has important bearings on Phylogeny, or the history of the race or group, which constitutes one of the most important branches of Zoology.
Were it indeed the case that each organism contained in its development a full record of its origin, the problems of Phylogeny would be in a fair way towards solution. As it is, however, the law above enunciated is, like all physical laws, the statement of what would occur without interfering conditions. Such a state of things is not found in nature, but development as it actually occurs is the resultant of a series of influences of which that of heredity is only one. As a consequence of this, the embryological record, as it is usually presented to us, is both imperfect and misleading. It may be compared to an ancient manuscript with many of the sheets lost, others displaced, and with spurious passages interpolated by a later hand. The embryological record is almost always abbreviated in accordance with the tendency of nature (to be explained on the principle of survival of the fittest) to attain her ends by the easiest means. The time and sequence of the development of parts is often modified, and finally, secondary structural features make their appearance to fit the embryo or larva for special conditions of existence. When the life history of a form is fully known, the most difficult part of his task is still before the scientific embryologist. Like the scholar with his manuscript, the embryologist has by a process of careful and critical examination to determine where the gaps are present, to detect the later insertions, and to place in order what has been misplaced.
1 Huxley was the first to shew that the body of the Coelenterata was formed of two layers, and to identify these with the two primary germinal layers of the Vertebrata.
The aims of Comparative Embryology as restricted in the present work are two-fold: (i) to form a basis for Phylogeny, and (2) to form a basis for Organogeny or the origin and evolution of organs. The justification for employing the results of Comparative Embryology in the solution of the problems in these two departments of science is to be found in the law above enunciated, but the results have to be employed with the qualifications already hinted at ; and in both cases a knowledge of Comparative Anatomy is a necessary prelude to their application.
In accordance with the above objects Comparative Embryology may be divided into two departments.
The scientific method employed in both of these departments is that of comparison, and is in fact fundamentally the same as the method of Comparative Anatomy. By this method it becomes possible with greater or less certainty to distinguish the secondary from the primary or ancestral embryonic characters, to determine the relative value to be attached to the results of isolated observations, and generally to construct a science out of the rough mass of collected facts. It moreover enables each observer to know to what points it is important to direct his attention, and so prevents that simple accumulation of disconnected facts which is too apt to clog and hinder the advance of the science it is intended to promote.
In the department of Phylogeny the following are the more important points aimed at
(i) To test how far Comparative Embryology brings to light ancestral forms common to the whole of the Metazoa.
Examples of such forms have been identified by various embryologists in the ovum itself, supposed to represent the unicellular ancestral form of the Metazoa : in the ovum at the close of segmentation regarded as the polycellular Protozoon parent form : in the two-layered gastrula, etc., regarded by Haeckel as the ancestral form of all the Metazoa 1 .
(2) How far some special embryonic larval form is constantly reproduced in the ontogeny of the members of one or more groups of the animal kingdom ; and how far such larval forms may be interpreted as the ancestral type for those groups.
As examples of such forms may be cited the six-limbed Nauplius supposed by Fritz Miiller to be the ancestral form of the Crustacea ; the trochosphere larva of Lankester, which he considers to be common to the Mollusca, Vermes, and Echinodermata : the planula of the Ccelenterata, etc.
(3) How far such forms agree with living or fossil forms in the adult state ; such an agreement being held to imply that the living or fossil form in question is closely related to the parent stock of the group in which the larval form occurs. It is not easy to cite examples of a very close agreement of this kind between the larval forms of one group and the existing or fossil forms of another. The larvae of some of the Chaetopoda with long] provisional setae resemble fossil Chaetopods. The Rotifers have many points of resemblance to the trochosphere, especially to that form of trochosphere characteristic of the Mollusca. The Turbellarians have some features in common with the Ccelenterate planula. Some of the Gephyrea in the presence of a praeoral lobe resemble certain trochosphere types. The larva of the Tunicata has the characters of a simple type of the Chordata.
Within the limits of a single group agreements of this kind are fairly numerous. In the Craniata the tadpole of the Anura has its living representative in the Pisces and perhaps especially in the Myxinoids. The larval forms of the Insecta approach Peripatus. The stalked larva of Comatula is reproduced by the living Pentacrinus and Rhizocrinus etc.
1 The value of these identifications as well as of those below is discussed in its appropriate place in the body of the work. Their citation here is not to be regarded as necessarily implying my acceptance of them.
Numerous examples of the same phenomenon are found amongst the Crustacea.
(4) How far organs appear in the embryo or larva which cither atrophy or become functionless in the adult state, and \vhich persist permanently in members of some other group or in lower members of the same group. Cases of this kind are of the most constant occurrence, and it is only necessary to cite such examples as the gill-slits and Wolffian body in the embryos of higher Craniata to illustrate the kind of instance alluded to. The same conclusions may be drawn from them as from the cases under the previous heading.
(5) How far organs pass in the course of their development through a condition permanent in some lower form. Phylogenetic conclusions may be drawn from instances of this character, though they have a more important bearing on Organology than on Phylogeny.
The considerations which were used to shew that the ancestral history is reproduced in the ontogeny of the individual apply with equal force to the evolution of organs. The special questions in Organology, on which Comparative Embryology throws light, may be classified under the following heads.
(1) The origin and homologies of what are known as the germinal layers; or the layers into which the embryo becomes divided immediately after the segmentation.
(2) The origin of primary tissues, epithelial, nervous, muscular, connective, etc., and their relation to the germinal layers.
(3) The origin of organs. The origin of the primitive organs is intimately connected with that of the germinal layers. The first differentiation of the segmented ovum results in the cells of the embryo becoming arranged as two layers, an outer one known as the epiblast and an inner one as the hypoblast. The outer of these forms a primitive sensory organ, and the inner a primitive digestive organ.
(4) The gradual evolution of the more complicated organs and systems of organs.
This part of the subject, even more than that dealing with questions of Phylogeny, is intimately bound up with Comparative Anatomy; without which indeed it becomes quite meaningless.
A study of reproduction logically precedes that of Embryology. Reproduction essentially consists in the separation of a portion of an organism which has the capacity of developing into a form similar to that which gave it origin. The simplest modes of reproduction are those which occur amongst the Protozoa.
In this group, reproduction may take place in a great variety of ways. These may be classified in three groups: (i) fission, (2) budding or gemmation, (3) spore formation.
Reproduction in all these ways may take place either subsequently to and apparently in consequence of a very important process known as conjugation, which consists in the temporary or permanent fusion of two or more individuals, or spontaneously, i.e. independently of any such previous conjugation.
Reproduction by fission consists simply in the division of the organism into two similar parts, the nucleus when present becoming divided simultaneously with the cell body. This mode of reproduction is the simplest conceivable, and is not followed by a development, since the two organisms produced are exactly similar, except in size, to the parent form. Besides single fission, a process of multiple fission may take place, as amongst the Flagellata, where Drysdale and Dallinger have shewn that an individual enclosed within a structureless cyst may divide first into two, then into four, and so on.
The process of budding differs mainly from that of simple fission in the fact that the two organisms produced are dissimilar in size, and also that the separation of the smaller organism from the larger is preceded by a process of growth in the latter, so that in the separation of the bud no essential part of the parent form is removed. This mode of reproduction is found amongst the Infusoria, Acineta, &c. An interesting variation in it is the internal gemmation of many of the Acineta, where a portion of the internal protoplasm with part of the nucleus is separated off to form a fresh individual. This mode of gemmation is connected by a series of gradations with the normal external gemmation. The organisms produced by gemmation are not always similar at birth to the parent ; e.g. Acineta.
Both fission and gemmation when incomplete lead to the formation of colonies.
The third mode of reproduction, by spore formation, does not essentially differ from that by multiple fission. It consists in the breaking up of the organisms into a number (usually very considerable) of portions ; each of which eventually developes into an organism like the parent form. All gradations between a simultaneous division of the organism into such spores and simple multiple fission are to be found, but this process of reproduction may be sometimes distinguished from that by such fission by the fact that the two processes may coexist in a single form, e.g. the biflagellate monad of Drysdale and Dallinger. In the majority of cases the spores produced differ at first from the parent organism not only in size but in other points, such as the possession of a flagellum, etc. They may even be without a nucleus when the parent organism is nucleated, as in the Gregarinidae.
The encystment, which in many cases precedes reproduction by any of the above processes, and more especially by spores, is not an essential condition of their occurrence ; and is probably in the first instance a protective arrangement which has become secondarily adapted to and connected with reproduction.
As has been already stated, all the above modes of reproduction take place in some of the Protozoa without any anterior process which can be regarded as of a sexual nature ; but very often they are preceded by the temporary or permanent fusion of two or more individuals, such fusion being known as conjugation.
In most cases reproduction by spores is the consequence of conjugation, but in the Infusoria etc. where the fusion at conjugation is temporary (except Vorticella), there is probably merely a renewed activity a rejuvenescence which most likely results in active fission or budding. In the Gregarinidse reproduction by spores usually follows conjugation, but may also take place without it. In some Flagellata reproduction by spores follows the conjugation of two individuals in a different stage of development. Thus in the springing Monad, described by Drysdale and Dallinger, a form produced by the fission of a monad in an amoeboid condition fuses with an ordinary monad to produce an individual, which then breaks up into spores. Another instance of the fusion of dissimilar individuals is afforded by Vorticella, where a free-swimming individual conjugates and is permanently united with a fixed one (Engelmann, Biitschli). Conjugation often consists in the fusion of more than two individuals. In conjugation where the fusion is permanent, the nuclei of the conjugating forms usually unite before the product breaks up into spores ; and where temporary fusion occurs in the Infusoria a division of the paranuclei and often of the nuclei takes place, followed by the ejection of parts of them, and a reproduction of new paranuclei and nuclei from the remainder of the original structures.
In order to understand the meaning of conjugation in connection with reproduction, it is important to understand how the two became in the first instance related. For the solution of this question the fact that many Protozoa have the capacity of temporarily or permanently fusing together without an immediate act of reproduction is of great importance. A good example of such fusion is supplied by Actinophrys. We must suppose in fact that the simple coalescence of two or more individuals gives a sufficient amount of extra vigour to their product, to compensate the race for the loss in number of individuals so caused. This extra vigour probably first exhibited itself especially by increased activity in reproduction, till finally the two processes, viz. that of conjugation and that of reproduction, came to be inseparably connected together.
The reproduction of the forms above the Protozoa, which are known as the Metazoa, takes place by two methods, viz. a sexual and an asexual one. The sexual process, which occurs in every known Metazoon 1 , consists essentially, as is shewn in the second chapter of this work, in the fusion of two cells budded off from the parent organism, viz. the female cell or ovum, and the male cell or spermatozoon, and of the subsequent division of the compound cell so produced into a number of parts which build themselves up into an organism resembling one of the parents. The sexual process has obviously at first sight a very close resemblance to the process of conjugation. Since it is a question of fundamental importance to determine how sexual reproduction originated, it becomes necessary to examine how far this apparent resemblance is a real one, and how far sexual reproduction can be derived from reproduction following upon conjugation.
1 Dicyema, if it is a true Metazoon, would seem to form an exception to this rule.
In spite of the general similarity between the two processes there is an obvious difficulty in comparing them, in that the result of conjugation is usually the breaking up of the individual formed by the fusion of two other individuals into a number of new organisms, while the result of the fusion which takes place in sexual reproduction is the formation of a single new organism. This difference between the two processes, great as it is, is perhaps apparent rather than real. It must be remembered that a single individual Metazoon is equivalent to a number of Protozoa coalesced to form a single organism in a higher state of aggregation. It results from this that the segmentation of the ovum which follows the sexual act may be compared to the breaking up of the product of conjugation into spores, the difference between the two processes consisting in the fact that in the one case the spores separate each to form an independent organism, while in the other they remain united and give rise to a single compound organism.
If the above considerations are well founded it seems permissible to accept the general view according to which sexual reproduction is derived from conjugation. It is necessary to suppose that, in a colony of Protozoa in the course of becoming a Metazoon, the capacity of reproduction by spores became localized in certain definite cells, and although the formation of spores from these cells may have been possible without previous conjugation, yet that conjugation gradually became established as the rule. The differentiation of primitively similar conjugating cells into male and female cells was probably a very early occurrence, since indications of an analogous differentiation, as has already been mentioned, are found in certain existing Protozoa (Monads, Vorticella, etc.). I have attempted to shew in the second chapter that the breaking up of the cell into spores without previous conjugation is perhaps provided against in the extrusion of the so-called ' directive body '.
With the differentiation of special germinal cells, to take the place of the whole individual in the act of conjugation, the possibility of each act of conjugation resulting in the production of only a single organism became introduced. Germinal cells can be indefinitely produced, and the reproductive capacity of a single individual is therefore unlimited ; while if two whole individuals conjugated and only produced one from the process, the result would be a diminution instead of an increase in the race 1 .
It must be admitted that, in the present state of our knowledge, the passage from reproduction by spores following conjugation, to true sexual reproduction, can only be traced in a very speculative manner; and that a further advance in our knowledge may prove that the steps which I have attempted to sketch out are far from representing the true origin of sexual differentiation. The peculiar conjugation and fusion of two individuals to form Diplozoon paradoxum may be alluded to in this connection. This fusion merely results in the attainment of sexual maturity by the two conjugating individuals. It does not appear to me probable that this conjugation is in any way connected with the conjugation of the Protozoa, but the reverse must be borne in mind as a possibility.
1 In the vegetable kingdom there are numerous types of Thallophytes, which throw a considerable amount of light on the relation between sexual reproduction and conjugation. Subjoined are a few of the more striking cases. In Pandorina at the time of sexual reproduction the cells which constitute a colony divide each into sixteen, and the products of their division are set free. Pairs of them then conjugate and permanently fuse. After a resting stage the protoplasm is set free from its envelope after division into two or four parts. Each of these then divides into sixteen coherent cells and constitutes a new Pandorina colony. In CEdogonium the fertilization is effected by a spermatozoon fusing with an oosphere (ovum). The fertilized oosphere (oospore) then undergoes segmentation like the ovum of an animal; but the segments, instead of uniting to form a single organism, separate from each other, and each of them gives rise to a fresh individual (swarm-spore) which grows into a perfect CEdogonium. In Coleochaete the impregnation and segmentation take place nearly as in CEdogonium, but the segments remain united together, acquire definite cell walls, and form a single embryo. There is in fact in Coleochaete a true sexual reproduction of the ordinary type. ( Vide S. H. Vines "On alternation of generation in the Thallophytes." Journal of Botany, Nov., 1879.)
It is not easy to decide whether the hermaphrodite or the dioecious state is the primitive one, or in other words whether the two conjugating cells, from which I have supposed the sexual products to originate, were derived in the first instance from one or from two colonies of Protozoa. On purely d priori grounds it seems probable that they were originally formed in one colony, and that their derivation from two colonies or individuals was inaugurated when the spermatozoon became motile. There can be no doubt that the dioecious state is a very early one, and that the majority of existing cases of hermaphroditism are secondary.
The above considerations with reference to the male and female cells appear to indicate that they were primitively homodynamous ; a conclusion which is on the whole borne out by the history of their development.
Although the modes of reproduction amongst the Metazoa have been divided into the classes sexual and asexual, there is nevertheless one mode of asexual reproduction which ought to be classified with the sexual rather than with the asexual modes. I mean parthenogenesis, which consists essentially in the development of the ovum into a fresh individual without previous coalescence with the male element. This mode of reproduction, which has a very limited range in the animal kingdom, being confined to the Arthropoda and Rotifera, is undoubtedly secondarily derived from sexual reproduction. The conditions of its occurrence are discussed in the second chapter.
It is remarkable that in certain cases the absence of fertilization causes the production of males (Bees, a Saw-fly, Nematus ventricosus, etc.); more usually it results in the production of females only, and there are very often in the Arthropoda a series of successive generations of females all producing ova which develope parthenogenetically into females; eventually however, usually in direct or indirect connection with a change of food or temperature, or other conditions, ova are formed which give rise without fertilization both to males and females.
The true asexual modes of reproduction amongst the Metazoa consist of fission and gemmation. Gemmation is by far the most widely disseminated of the two. Various as are the methods in which it takes place, it seems nevertheless that cells derived from all the germinal layers, and very frequently from all the important organs of the adult, assist in forming the bud. Into the details of the process, which require in many points a fuller elucidation, it is not my purpose to enter.
Gemmation is a far commoner occurrence amongst the simpler than amongst the more highly organised forms. It appears to have been superadded to the sexual mode of reproduction quite independently in a number of different instances.
While there is no difficulty in understanding how gemmation may have started in such simple types as the Coelenterata, the manner in which it first originated in certain highly organised forms, as for instance the Ascidians, is somewhat obscure, but it seems probable that it began with the division of the developing germ into two or more embryos, at a very early stage of growth.
Such a division of the germ is, as has been shewn by Kleinenberg, normal in Lumbricus trapezoides 1 and Haeckel has shewn that an artificial division of the germ in the Siphonophora leads to the development of two individuals. It has been pointed out by various naturalists that the production of double monsters is often a phenomenon of the same nature. While it is next to impossible to understand how production of a bud could commence for the first time in the adult of a highly organised form, it is not difficult to form a picture of the steps by which the fission of the germ might eventually lead to the formation of buds in the adult state.
The coexistence of sexual reproduction with normal asexual multiplication, or with parthenogenesis, has led to a remarkable phenomenon in the animal kingdom known as alternations of generations 2 .
1 The case of Pyrosoma, which might be cited in this connection, is probably secondary.
2 For an excellent account of this subject, vide Allen Thompson's article Ovum in Todd's Cyclopaedia. The metamorphosis of the Echinoderms included under this head in Thompson's article is now known not to be a proper case of alternations of generations.
For the details of the various types of alternations of generations, and their origin, the reader is referred to the body of the work ; but a few general remarks on the nature and origin of the process, and on its nomenclature, may conveniently be introduced in this place. The simplest cases are those in which an individual which produces by sexual means gives origin to asexual individuals differently organised to itself, which produce by budding the original sexual form, and so complete a cycle. Instances of this kind are supplied by the Hydrozoa, Annelida and Tunicata. In the case of the Tunicata (Doliolum) two different asexual generations may be interpolated between the sexual generations. In all these cases the origin of the phenomenon is easily understood. It appears, as is most clearly shewn in the case of the Annelida, that the ancestors of the species which now exhibit alternations of generations originally reproduced themselves at the same time both sexually and by budding, though probably the two modes of reproduction did not take place at the same season. Gradually a differentiation became established, by which sexual reproduction was confined to certain individuals, which in most instances did not also reproduce asexually. After the two modes of reproduction became confined to separate individuals, the dissimilarity in habits of life necessitated by their diverse functions caused a difference in their organization ; and thus a complete alternation of generations became established. The above is no merely speculative history, since all gradations between complete alternations of generations and simple budding combined with sexual reproduction can be traced in actually existing forms.
The alternation of generations as it is found amongst the Entoparasitic Trematodes and most Cestodes, is to be explained in a slightly different way.
It appears that in these parasitic forms a complicated metamorphosis first arose from the parasite having to accommodate itself to the different hosts it was compelled to inhabit, owing to the liability of its primitive and subsequent hosts to be devoured 1 . A capacity for asexual multiplication obviously of immense advantage to a parasite appears to have been acquired in some of the stages of this metamorphosis, and an alternation of generations thus established.
1 The appearance of Vertebrata on the globe as the forms which most frequently preyed on Invertebrate forms, and were themselves not so liable to be devoured, has no doubt had a great influence on the metamorphosis of internal parasites, and has amongst other things resulted in these parasites usually reaching their sexual state in a vertebrate host.
A nearly parallel series to that exhibiting alternations of sexual generations with generations which produce by budding is supplied by the cases where sexual generations alternate with parthenogenetic ones, or in some instances even with larvae which reproduce sexually or else parthenogenetically.
The best known examples of this form of alternations of generations are found amongst the Insecta 1 . A simple case is that of the Aphides. The ova deposited by impregnated females give rise to forms differently organised to the parents but provided with an ovary 2 . The eggs from the ovary develope parthenogenetically within the oviduct, and so long as there is plenty of food and warmth the generations produced are always parthenogenetic forms. The failure of warmth and nutriment causes the production of true males and females, and so the cycle is completed. We must suppose that the capacity possessed by so many female insects of producing eggs capable of developing without the influence of the male element, has been, so to speak, taken hold of by natural selection, and has led to the production of viviparous parthenogenetic forms, by which, so long as food is abundant, a clear economy in reproduction is effected. The continuance of the species during winter is secured by the production of males and females, the females laying eggs in autumn which are hatched in the spring.
In Chermes there is less modification of the primitive condition in that the parthenogenetic generations lay their eggs like the impregnated females. In the gall-flies (Cynipidae), there is frequently an alternation of generations of the same kind as in Chermes ; there being no viviparous forms. The individuals of the different generations differ from each other to some extent in all these cases.
A second type of alternations of parthenogenetic and sexual generations is exemplified by the cases of Chironomus and Cecidomyia, where the larva which develope from the eggs of the fertilized female produce parthenogenetically, by means of true ova, forms which eventually after several generations (Cecidomyia) of larval reproduction give rise to sexual forms. The explanation is here practically the same as in the case of Aphis, and is paralleled in the gemmiparous series by the production of buds in the larval forms of Trematodes, etc. A very similar occurrence takes place in Ascaris nigrovenosa (vide chapter on Nematoidea), except that larval forms, which carry on reproduction and then perish without developing farther, do so by a true sexual process. Thus there is an alternation of generations of adult and larval sexual forms. The Axolotl is an intermittent example of the same phenomenon.
1 For details vide Chapter on Insecta.
2 The distinction drawn by Huxley between ova and pseudova does not appear to me a convenient one in practice.
As might be anticipated from the mode in which alternations of generations have become established, incomplete approximations to it are not uncommon. Such approximations are especially found in the Arthropoda, where alternations of sexual and parthenogenetic generations frequently take place, in which the individuals of different generations are similarly organised (Psychidae, Apus, &c.). Another approximation is afforded by the parthenogenetic winter eggs of Leptodora amongst the Phyllopods, which give rise to Nauplius larvae, while the young hatched from the summer eggs do not pass through a metamorphosis. Numerous transitional cases are also found amongst the forms in which there is an alternation of sexual and gemmiparous generations.
The whole of the cases to which allusion has been made in this section may be conveniently classed under the term alternations of generations, but the cases of alternation of two sexual generations, and of sexual and parthenogenetic generations, are classified by Leuckart, Claus, etc. as cases of heterogeny, which they oppose to the other form of alternation of generations. If special terms are to be adopted for the two kinds of alternation of generations, it would be perhaps convenient to classify the cases of alternations of sexual and gemmiparous generations under the term metagenesis, and to employ the term heterogamy for the cases of alternation of sexual and parthenogenetic generations.
The term Nurse (German Amme), employed for the asexual generations in metagenesis, may advantageously be dropped altogether.