Book - Outline of Comparative Embryology 2-4

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Richards A Outline of Comparative Embryology. (1931)
1931 Richards: Part One General Embryology 1 Historical Development of Embryology | 2 The Germ-Cell Cycle | 3 Egg and Cleavage Types | 4 Holoblastic Types of Cleavage | 5 Meroblastic Types of Cleavage | 6 Types of Blastulae | 7 Endoderm Formation | 8 Mesoderm Formation | 9 Types of Invertebrate Larvae | 10 Formation of the Mammalian Embryo | 11 Egg and Embryonic Membranes | Part Two Embryological Problems 1 The Origin And Development Of Germ Cells | 2 Germ-Layer Theory | 3 The Recapitulation Theory | 4 Asexual Reproduction | 5 Parthenogenesis | 6 Paedogenesis And Neoteny | 7 Polyembryony | 8 The Determination Problem | 9 Ecological Control Of Invertebrate Larval Types

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Chapter IV Asexual Reproduction

Asexual reproduction occurs in a great many plants and in many animal groups. Among the invertebrate phyla it is lacking only in the arthropods, molluscs, and nematodes of the more important groups; while among the chordates, the tunicates present many illustrations of this mode of reproduction. It should also be noted in passing that even in certain mammals a process of budding in the embryonic state occurs. This matter is discussed at length in the chapter on polyembryony where it is shown that the blastocyst of the armadillo regularly buds to produce four embryos. This gives asexual reproduction a place even among the highest groups.

In spite of the fact that this method of reproduction manifests itself in many different forms, it is essentially a very simple process. It occurs in organisms (or in portions of organisms) which have retained to a considerable extent their embryonic, undifferentiated character, and is really a mass division, due especially to the cells of some particular area undergoing a proliferation which presently results in the constriction and cutting ofi” of a greater or lesser portion of the animal. If the portions are approximately equal the process is spoken of as fission; if unequal, as budding. These, with sporulation, are the common forms of asexual reproduction, but each shows many variants in the animals in which it occurs.

Sexual reproduction, involving the participation of two individuals, is known as amphigony; asexual, since only a single individual is necessary, is monogony. In the former, germ cells or their equivalents are produced; in the latter, there are no special cells employed for the purpose. The alternation of a sexual with an asexual generation is called metagenesis. The alternation of a biparental sexual generation, that is, a case of amphigony, with a uniparental, a parthenogenetic, generation is heterogony. Sexual reproduction was further called gamocytogony or cytogony by Hartman, and asexual, agamocytogony (also agamogony) fission is the division of an organism into two parts which are approximately equal and into whose formation very little new material has gone. The parent organism is lost in the production of the daughters, which therefore can have no living ancestors, and can undergo only what may be termed accidental death. It occurs when growth has taken place in excess of the needs of the individual. Contrasted to the condition in sexual reproduction, the new organism is at once provided with at least part of the organs which characterize the adult, and differentiation is always well advanced, considering that the organism is seldom high in the scale of animal life. Two categories were distinguished by F. von Wagner for organisms undergoing fission: in the one, paratomy, a special zone, in which the constriction will occur, is prepared before fission begins; in the other, architomy, no preparation is made, constriction taking place more primitively with little bodily reorganization. It should be noted that both of these categories may apply to either longitudinal or cross fission. Under the general head of fission may also be placed those cases of fragmentation, such as occur in oligochaetes and starfish, due to external influences, which may, if conditions are suitable, develop into mature individuals. These cases are also spoken of as autotomy, and sometimes as augmentation.

Budding, although not much more complex as a process than fission, is productive of much more complicated organisms and life cycles. It often is the means by which colonies are formed, and results in marked polymorphism with some of the individuals of the colony being specialized for nutritive and vegetative purposes and others for reproduction. Commonly where a considerable degree of specialization follows the budding processes the life cycle includes a sexual as well as an asexual phase and we have alternation of generations. A bud is a small portion of the parent organism which has begun to grow actively and to proliferate and from which will be derived a new individual having the full degree of differentiation characteristic of the species. There are buds in some triploblastic animals which involve only one of the germ layers, whereas in other animals the buds may include more than one layer. Furthermore budding may occur in embryonic, larval, or mature animals. Abundant illustrations of all these cases may be cited.

There are several types of budding. The most familiar are the external buds such as occur in Hydra for example in which a small portion of the parental tissue grows and constricts off to become a new, small hydra. In some of the hydroids, however, the separation is incomplete and the new individual remains as a permanent bud. The so-called “free buds” of certain forms are also of the external type. As buds they break off and become for a time at least free swimming. A second type includes the internal buds among which are the gemmulae of sponges, the statoblasts of the bryozoa, the germ balls of trematodes and others. A group of proliferating cells becomes isolated within the mother organism and in time results in a new organism. A third type involves the formation of a stolen or a “runner” from which numerous buds arise. This might seem to be really a kind of external budding but it is sufficiently distinct from the usual cases of this kind to warrant special mention. finally a fourth series of processes should be included under budding, although they are often overlooked; they are the processes known as frustulation and laceration in which small fragments separate off from the parent organism, when unfavorable conditions arise, and form new individuals.

Closely related to asexual reproduction and of much significance to the comparative embryologist are the phenomena of regeneration. Morphogenic processes are involved in the reorganization of portions of the old individuals to produce new ones which do not greatly differ from the preceding. These processes involve the regeneration of the organism or of portions of it. The student of comparative embryology should give careful thought to these processes, for in them are manifested fundamental capacities and characteristics of living protoplasm. Indeed some of these processes take us far into the innate organization and give an insight into the nature of living stuff which the Inorc usual studies fail to offer.

The formation of colonies is closely correlated with the occurrence of asexual reproduction. Colony formation occurs in certain phyla in a striking manner, and it is in these very phyla that asexual reproduction is a dominant method. It may be taken as a general rule (not, however, without exception), that whenever individuals are found organized into colonies, either asexual reproduction, or polyembryony, or parthenogenesis will be found to occur in the same groups. The simple sexual mode of reproduction seems not usually adequate to produce enough individuals to be associated together in a permanent colony, and one of those three accessory modes must be depended upon for the increased task.


The first examples of asexual reproduction to be found in the animal kingdom are in the protozoa, but the process here is one of single cells and perhaps is thus somewhat outside the general problems of embryology. We may begin our study therefore with the Porifera.

Porzfera. In the phylum Porifera, asexual reproduction predominates; here are to be found fission, budding, formation of free buds, and of gemmulae. Since most sponges exist as colonies it is easy to see how the dividing processes have lacked completion resulting in a degree of union that is more or less extensive. Asexual reproduction commonly leads to formation of colonies rather than to independence of organisms. But budding and fission are shown to advantage in Leucosolenia. Some of the individuals indicate fission as their mode of reproduction for the products are equal in size although they remain attached at the basal end. However, the presence of small immature individuals on certain

Fm 194 Budding and fission in Leucosolema blanca. (From Korsehelt and Heider)

specimens as well as the mode of colonial formation shows that budding is often the actual method of their origin. When fission does occur it begins as a split at the osculum and progresses toward the base. When

fiG. 195. A, Budding in Leucosolema botryozdes B, A bud which has become free and attached to an algal filament. (From Korschelt and Heider, after Vasseur.)

it is not complete the beginning of a colony is seen and these colonies

often become quite complex. In another species of Leucosolenia (botrymdes), Vasseur long ago found free bud formation. The buds form from an indifferent group of cells, grow irregularly from the parent individual, undergo differentiation to a considerable extent, and at length break off, the ruptured end forming the osculum of the new little sponge; they then settle down to produce young sponges.

The sponges show what is perhaps their highest degree of asexual reproductive activity in the formation of gemmulae, seen to best advantage in the Hexactinellidae. They are derived from parenchymal cells which have wandered into the mesoglea and become separated from their original layers. Aggregates of these “archeocytes” (also called “sorites” by some) take on an oval or rounded shape, become

I‘it. 106 (:(‘lllI‘n1ll( forination in Eph;/dalm blrmbmata (From Korschelt and Heider. Liter Evans )

A, an early stage showing the aggregation of the ‘ germ" cells (g) from those which make up the covering membrane (m) B, the cutieular membrane (c) is beginning to form from the outer membrane

surrounded by a special membrane and thus make up a gemmulc. During conditions which are not favorable, this structure, which is really an internal bud with a protective covering, tides over the organism until a more suitable time. Thus this form of reproduction is a device which enables these sponges to adapt themselves to changing environments. The parental tissue degenerates and dies after the gemmules are formed. When the gemmules begin to grow, the cells multiply rapidly, those at the surface arrange themselves in a layer, and from the gemmules a larva issues which is strikingly similar to the sexually produced young in form, structure, and ciliation.

Division among sponges is often accidental so far as the organism is concerned, and the animals offer opportunity for experimental fragmentation as well as normal. H. V. Wilson’s work and that of Huxley give illustrations of the extent to which sponges may be dismembered (in these cases by being squeezed through bolting cloth) and subsequent regeneration serve to produce new sponges. Often cell masses come together, fuse, and grow into a new individual from which a colony is formed. This is called concrescence and may also occur between larvae as they creep about on the bottom.

Coelenterata. By the eoelenterates many developmental experiments along the lines of asexual reproduction seem to have been tried as well as those which have already been shown for the various types of embryos, cleavage patterns, and methods of gastrulation. Nearly all types of asexual reproduction are exhibited somewhere in the phylum. In addi

Fm. 197. Trunsversc fission in Protohydra. (From Korschclt and Heider. ufter Aders.)

tion there is often manifested an extreme polymorphism in the forms which are thus produced, and the details of the manner of their production especially where a compound type of budding is involved are often extremely complicated. Mention here can be made of only a few cases which constitute a very meager outline indeed. fission, both transverse and longitudinal, budding both larval and adult, and colony formation by the production of permanent buds, metagenesis, stolen formation, fragmentation, frustulation, laceration, and the related, although in its results opposing, process of concrescence, occur in this group.

The Hydrozoa exhibit only a few types of asexual reproduction. Transverse fission occurs in Protohydra, according to Aders, and consists in the simple constriction of the animal around the region of its greatest diameter. The constriction cuts the animal into a proximal and a distal portion. The one develops a new base, the other a new oral region. A similar type of division may occur in Hydra; although undoubtedly very rare, this was one of the earliest cases to be described (Trembly, 1744; see also Koelitz, 1908). It is of essentially the same character as in Protohydra, the chief points of difference between the two being in the simpler structure and the lack of tentacles on the part of the latter. Longitudinal fission has been described for Polypodium only among hydrozoa.

Budding by larval as well as by adult hydroids is not uncommon. An example of budding in the larval condition occurs in Gonioncmus and in Ilaleremita as described by Schaudinn, Perkins, and others; the details of the life cycle in these forms have been lately worked out by Joseph. The egg hatches into a tiny planula larva. It develops a mouth and creeps about actively while feeding in Haleremita, although the planula of Gonionemus is sessile. On the sides of these larvae protuberances appear which grow into buds that gradually elongate to become like the planula, then constriet at their bases, and at length separate. The descriptions of the processes differ greatly in detail as described by the different investigators. For example, from one to six buds have been reported by the various students, according to Joseph, near the base of the larva, although Perkins found them about the middle rather than the base. In some cases the endoderm of the bud is said to be solid, in others to contain a cavity derived from the gastrovaseular cavity of the mother larva. There are also other differences in the details as given, some of which are doubtless to be attributed to the fact that both European and American forms have been used by the investigators. Lateral buds also have been described as occurring on Microhydra, where they may form tiny colonies of three or four polyps.

Among adult hydrozoans budding occurs in both polyps and medusae. In Hydra and the colonial hydroids it is of such common occurrence that mere reference to it is suflicient. The bud of Hydra consists of a protuberance which grows, develops tentacles, a hypostone, and at length a mouth, after which it is ready for separation from the parent. Both layers of the body wall participate in the formation of the bud, and the gastrevascular cavity is continuous between parent and bud until the time of complete constriction of the latter. Budding is much more common than sexual reproduction. In hydroids the process is essentially the same except that it is usually incomplete, the bud remaining in connection _with the parent stalk; in this manner a colony is formed.

A special case of hydroid budding is that which gives rise to the medusa which is morphologically the equivalent of a bud from a polyp. There are cases, to be sure, where the ontogeny is so abridged that the polyp stage is extremely rudimentary or even entirely lacking, unless the planula be regarded as representing it, and the egg develops continuously into the medusa. But in general the medusoid generation is produced by budding from the hydroid generation or the polyp. The main stalk, or hydrocaulus, has attached to it in Obelia, modified hydranths, called gonangia, which bud to produce medusae. In Obelia these become detached to swim freely. In the Narcomedusae a proliferating stolon buds off medusae which may remain in clusters or may separate off completely. In the siphonophore, Halistemma, the planula develops an ectodermal thickening at the aboral pole which develops into the pneumatophore or float. Part of the planula becomes the eoenosarcal axis from which spring buds which become several different kinds of individuals. Those near the float are bell-shaped medusae, through whose efforts the colony is enabled to swim. Next come a series of covering scales which seem to be retrogressed medusae; they are protective in function. At various places along the coenosarc are feeding tubes in general similar to a hydranth. The tentacles and feelers also are probably to be looked upon as hydranths. finally there are the reproductive individuals which resemble certain types of medusae.

It will be seen from a consideration of all these cases that budding in the hydrozoa directly results in the formation of colonies. All hydroids which bud form colonies. Usually the planula develops directly into the first hydranth and the colony is formed by the subsequent branching and budding. This brings about in all except the simplest cases a very considerable degree of polymorphism. There are defensive and sensory individuals and in these the cnidoblasts are well developed. The gonanth or gonangium has for its purpose the function of reproduction, and various modifications of gonangia are to be found throughout the group. Medusae which are free swimming show a greater degree of differentiation than any of the other types of individuals produced in a hydroid colony. There are, however, numbers of medusa forms in which the development is incomplete, and some of them are quite simple in structure. As an accompaniment to the more complex polymorphism which we find in these colonies metagenesis, the alternation of generations, is perhaps shown here to a degree of completeness that scarcely exists elsewhere in the animal kingdom. The sexual functions are transferred to certain individuals while others specialize along the lines of feeding and protecting the colony. Hydroid colonies not uncommonly reproduce by another asexual method, namely stolonization. From the hydrorhiza of the original polyp cylindrical projections grow out which elongate, creep about on the bottom and may branch or even anastomose. These are stolons or runners upon which new hydranth buds appear to produce new individuals. Bougainvillia and Clavularia both serve as illustrations of colonies which grow by stolon formation.

In the second class of coelenterates, the Anthozoa, transverse fission occasionally occurs in young animals which have not yet developed sex organs. In Fungia as described by Bourne, the development of the larva at a certain stage results in the separation of its distal part and after a complicated series of processes two individuals are produced. Longitudinal fission is common among the Anthozoa. It is a slow process beginning at the oral pole in some cases, although in Actinia it may take place simultaneously from the aboral as well. In the adult Sagartia and in Paranemonia a constriction begins at the pedal disc and passes in the course of twenty-four hours to the oral region. In some others it is much more rapid. Furthermore the constriction of one individual to produce several at the same time may take place. In this case the resulting individuals will be of varying size, and it has been observed that one division may not even be completed before a new one begins. Strange complications thus arise in which the individuals may have several mouths or several systerrs of septa at the same time owing to this multiple fission. In the Anthozoa irregular longitudinal fission sometimes gives the appearance of budding. True budding, however, is rarely met with in the sea anemones. In the alcyonarians, however, much-branched colonies are formed by budding and the individuals undergo modification and even produce a very considerable polymorphism. The Zoantharia likewise owe their extremely complicated type of development to both fission and budding. In both these latter groups stolon formation and subsequent budding are commonly observed.

One of the most striking forms of asexual reproduction is that manifested by the scyphozoa in the process of strobilization. This has already been described in the chapter on “Types of Invertebrate Larvae.” The planula develops into a hydranth-like form called the scyphistoma. By a series of divisions which may be repeated perhaps a dozen times there are constricted off from the scyphistoma, ephyra larvae of which there may be one or several. If there is only a single ephyra strobilization is said to be monodiscal, if several are produced it is polydiscal. The question arises as to the nature of these divisions. The first ephyra is commonly said to be formed by terminal budding and it would seem that if the successive ones are produced only slowly they likewise are terminal buds. However, in certain well-fed scyphistomae the process of strobilization takes place so rapidly that the lower individuals are already indicated by constrictions before the upper ones have progressed to any considerable degree of independence. It would seem that the distinction between terminal budding and transverse fission in this case is a difligult one to draw. The scyphistoma also for a considerable part of the year produces other scyphistomae by lateral budding in a manner similar to that in Hydra. All at length, however, undergo strobili— zation and produce ephyrae which gradually develop into adult jellyfish. In certain scyphozoans another type of asexual reproduction occurs in the formation of stolons from the original hydranth. They grow out from its base but remain in connection with it and their buds form a colony.

Before leaving the coelenterates the attention should be called to the other methods of asexual reproduction already discussed, namely, fragmentation or laceration and frustulation. In the actinians laceration is rather frequently seen, especially when the conditions of the water become unfavorable. A part of the basal rim begins to spread out, the ectoderm develops quite profusely and endodermal portions grow out into this new area. This then separates from the main body owing to the contractions of the latter and from the pieces so produced new anemones may at length regenerate, although the ability to do so is dependent upon the number of septa present. Frustulation occurs in the hydroids. Occasionally a small bud-like branch is observed to constrict off from the parent colony, to settle down on the bottom in which condition it is called the frustulum, to grow and to produce a hydranth. In hydranths also a process somewhat similar to laceration has been observed, although in this case it is spoken of as fragmentation, for a basal portion of the polyp is cut off and may develop into a young polyp.

A final question in connection with the coelenterates naturally arises as to which condition is the more primitive. There are three possibilities: one, that budding is the primitive type of asexual reproduction; two, that fission is primitive; and three, that both arose independently of each other. There have been interesting discussions of this question and transitions between them have been pointed out. It need only be said here that the evidence is not conclusive for either view.

Platyhelminthes. In the phylum Platyhelminthes the dominant type of asexual reproduction is fission, although there are also causes of budding. It is most common among the Turbellaria, although the polyclads have not been shown to reproduce asexually. Undoubtedly the phenomenon of asexual reproduction in this division of the animal kingdom is closely concerned with the ability which all members of the phylum have for extensive regeneration. In its simplest form, as for example, in the triclads, Planaria abissima and P. alpina, the reproduction really consists in the separating of the animal by transverse fission into two portions each of which proceeds to regenerate the missing head or tail. In Planaria all the species described show that regeneration may take place at various levels proceeding from the anterior to the posterior. An area of regenerating tissue appears which enlarges and then differentiates. Sometimes a second fission makes its appearance before the complete regeneration following the first, and indeed there are recorded cases of several divisions with the appropriate organs already developing before the first is completed.

Among the rhabdocoels fission is an even more general phenomenon. The small fresh-water M zcrostoma reproduces mainly by fission. Commonly this process occurs before the young have developed sex organs in this form and in Stentostoma, and the divisions may take place in such rapid succession that chains of individuals incompletely divided are the result. Division takes place by paratomy very slowly, that is, a zone of division is prepared and the organs of the new portion of the animal are already developed to a considerable extent when constriction occurs. If the constrictions follow each other more rapidly they come under the category of architomy, no special region being A prepared in advance. The distinction pm 195

S\l(‘('(“§‘\l\(‘ lI‘f'lH§V€l'EaC fis betwcon pal-altolny and 3'1-chit/Oxny sions 111 (A) Stuwstmnum SM-boldi, and

. (B) '11 trrnslomum lmcarr (From Kormay perhaps be made clear by Saylng schelt and Holder after v Grofi )

that the regeneration Of the new Pharvnx is shown for each single inorgans precedes the actual fission in dividual, and the dn 131 111 planes of the

_ different ranks are numbered paratomy, no great change in the

organization of the animal taking place at the exact moment of division. On the other hand, in architomy, regeneration and subsequent reorganization follow the constriction.

In general, asexual reproduction in the trematodes is rare and certainly never occurs in any individuals which have a complete set of sex organs. The situation with regard to the germ balls which produce the rediae has already been referred to in connection with the chapter on in vertebrate larvae and also that on polyembryony. It is still looked upon as an open question as to whether parthenogenesis or asexual reproduction is the method by which the germ ball that forms the new rediae is produced. It has been pointed out that originally they were regarded as asexually produced, more recently as developed in parthenogcnie ova, and finally that the cytological study of these supposed ova has not yet shown evidence of a reduction division such as would be necessary to establish surely the fact of their parthenogenic nature. If we are to regard them as asexually produced then this type becomes of much more widespread occurrence among the trematodes than is usually considered.

Among the cestodes there are two aspects to the question of asexual reproduction. One has to do with the formation of the proglottids from the scolex, the other with multiplication of the eysticercus. The first of these involves a decision as to the fundamental nature of the proglottids. Is the tapeworm to be looked upon as a colony of individuals or as a single one? If the latter

fiG 199 * view is taken there eanof course be noquestion of asexual

fzvsli::;:::;;::)‘lf,? reproduction involved. If on the other hand the pro~ T‘”"“‘ "‘”““7’3 I ttid is re arded as an individual since each h‘ “ltll an accessory g 0 . ’ a v(..,,,.1e attached complete genital apparatus and since each has the ability S°°l°°°5 “"3 to live at least for some time after the separation from present in both _ _ (From Korschelt the remainder of the worm, then its method of forma;‘3';‘:tI){°'d°" “Mr tion must be looked upon as budding, it being thus

. produced from the scolex. It is the writer’s opinion that

the colonial view is not as generally held now as formerly.

There can be no question that in the development of the tapeworm from the eysticercus budding does occur. The egg develops into the onchosphere, the six-hooked embryo, in which condition it escapes from its adult host, and is taken in by the intermediate one. From it develops directly the eysticercus. The scolex of the eysticercus arises from a thickening in the wall of the bladder which becomes depressed into the cavity and later this bud-like structure is cverted to form the little worm.

It occasionally happens that two or more scoleces are formed by budPOLYZOA 307

like thickenings on the wall of the same bladder. This multiple production of scolices is rather rare but occurs regularly as is well known in Taenia echinococcus with disastrous results to the host. This is of course reproduction by budding. In some forms the bladders themselves may constrict and produce two vesicles each of which buds into a scolex. Polyzoa. Since the Bryozoa, or Polyzoa as it is now becoming custom~ ary to call them, commonly form colonies, it is to be expected that asexual reproduction will be found in this group, and the expectation is borne out. Three asexual methods, as well as polyembryony, are found here. They are budding, stolonization, and the formation of statoblasts. As a manifestation of the complicated condition which asexual reproduction may reach in a single group, the Bryozoa probably exceed any other branch of the animal kingdom. A student of the subject will find much to interest him in the group and may expect to see an everchanging variety of detail as he studies the different divisions of it. For

Fm. 200. Buds of ('ris!aleHa muredo in median section. (From Korschelt and Ileider. after Braem.) ee , ectoderm, m., mesoderm.

the general student of comparative embryology, however, it seems unnecessary to give more than a very brief consideration to the general types of asexual reproduction as they are developed here. From a consultation of the table of classification on page 260 the student will observe that the phylum Molluscoidea, of which the Polyzoa are a portion, consists of animals very diverse in structure and in their method of development. They range from rather simple colonies to aggregations of organizations which are complex in the extreme. Among the simplest genera is Cristatella, coming under the ectoproct order, Phylactolaemata. All the individuals of a colony of this form can be traced to the first one which develops from the larva. This individual produces buds commonly as thickenings of the pharynx on the oral side, both ectoderm and mesoderm participating in the process, although the ectoderm contributes a larger portion to the formation of the new individual. As the bud enlarges it grows out from the parent individual and its inner cell mass undergoes a very considerable differentiation. The intestinal tracts commonly remain in communication at least for some time. An entire series of buds which in their turn repeat the process is involved in the production of the colony. The development of buds in the Endoprocta closely resembles that just described. The bud is first an enlargement of eetoderinal cells which receives migrations of meso W

F16. 201. A piece of Plumalella fungnsa showing the formation of the primary buds and of those which arise secondarily.

dermal cells as it develops. In this group a stalk makes the bud rather more independent than in the preceding. In some forms, notably the ectoproet groups Crissia and Tubulipora, budding takes place in the embryonic stages even before any differentiation of the blastomeres begins. The primary embryo produces a

32 B13 C D E F great number of buds in such a case and B, may become a stem and retain the others as secondary embryos or it may be en— tirely destroyed in their production. Of course the secondary embryos may also reproduce asexually. It is rather rare that the buds should separate naturally from each other, for it is by their uninterrupted relation to each other that the colony is Fm 202. Dmgmmmntic “_pm_ formed. They may branch laterally from

aem,m.,n of the method 0; i,m,ci.- the main stalk or may continue in a direct

in“ line of the main branch. It is obvious that Korsehelt and Heider, after _ , . _ ’ Bmem) in such a highly organized type of animal as this, polymorphism of the individuals is to be expected. Special morphological changes take place so that some of the individuals of the colony become very much changed from the typical form and have very different functions. The Bryozoa. always reproduce by sending out stolons which then bud many times. The formation of the stolon itself is like that of the bud but it grows out as a stalk which

by repeated incomplete divisions produces a new portion of the colony.

Some fresh-water forms produce hibernacula. which.‘ are essentially winter buds, club-like swellings on the stolons which enclose themselves in a cutaneous capsule during the period of severe winter. In the spring their development continues to form new colonies. Accidental subdivision of a colony is of course common, but the multiplication of the colony by fragmentation appears also to occur naturally.

fiG. 203. Production of zooecia ns buds from stolons (st.) of Pedicellina erhinala. (From Korschelt and Heider, after Ehlors.)

A final method of asexual reproduction is the formation of statoblasts. According to an old view a statoblast is supposed to arise from a single cell and upon the basis of this it was thought to be a sort of winter egg. It is now realized that the statoblasts are modified buds which have become changed into reproductive buds. They vary in shape, but are enclosed in a cutaneous capsule and contain several cells. They are able to withstand extremes of temperature and thus serve to carry on the colony during the cold of the winter. Amwlida. Asexual reproduction is of common occurrence in the two most important groups of the annelids, namely the polyehaetes and the

Fm. 204. Statoblasts of Cristatella mucedo. (From Korschelt and Heider, after Kraepelin.)

oligochaetes. This is due to the remarkable uniformity in the structure which the annelid body possesses, each segment being very similar to its neighbor. The growth of the annelid worm is accomplished by the

F10. 205. Developmental stages of the statoblasts of Crietazella. (From Korschelt and Heider, after Verworn.)

addition of the somites to the posterior end primarily, although anterior zones of budding are also found. In some instances the formation of new somites takes place more rapidly than their separation from the animal ANNELIDA 31 1

and thus a chain of individuals is formed. In a simple annelid chain in which the individuals of a series are spoken of as zooids in anticipation

fiG. 206. Germinating statoblasts of Cristatella. (From Korschelt and Heidcr, after Braem.)

A. the “germ disc" stage. B, its invagination preparatory to further growth.

of their future development to individual worms the anterior individual represents the original worm and is obviously the oldest. At its posterior

fiG. 207. Posterior end of Trypanosyllis misakiensis showing buds of difierent ranks. (From Korschelt and Heidor. after Johnson.)

end is a reproductive zone where the budding oil’ of new individuals takes place. The first individual budded off is of course the most posterior of the chain, the others grading in age and size toward the middle of the worm where the budding zone is located. In the more complicated cases, of which the syllids are the extreme example, we may have not only terminal budding but also lateral and even ventral. In one of these forms, Trypanosyllis misakiensis, single budding soon produces numerous buds in all directions differing in age but ,.,:‘:.:-as *;:::r;..:" Merwisc giving the W231‘(Fmm Komheltamui ance of a rosette of small zooids. after v. Kenr}x]<:)l.) Between these two extremes lie gen‘: 0, S ‘V various degreesof complication in the forms produced. In many marine polychaetes a differentiation with respect to reproductive capacity is observed between the (lif ferent parts of the animal. The anterior portion of

the individual is sexless and is spoken of as the atoke. = H At the time of sexual maturity this slow-moving worm becomes very active, the hinder somites de- = 1

velop gonads, and special 1

‘W bristlesandparapodiagrow

El ‘ W _ out on them, developing

lllllllll them for rapid and exten sive movement. This sexual

portion is spoken of as the __

cpitoke, and individuals of "

this kind were earlier given

special systematic descrip tions. It is now clear, how ever, that N ereis, for ex— Fm- 209. Diu. grains to show the

ample; passes Over "Ito divisions zones in

H eteronereis or the epito— £0": hmlzaa. d(fir{or}1 - . . t kous stage. In many po1.v- dfflfnir c‘L'how..,°.3

chaetes, the epitokous por tion separates from the remainder and swims

Fm. 210 The mlom about a(t1vely for a certain perio a e sur

worm. Eunice vimkiis, show- face of the sea. This is the swarming which

5”‘! ‘he ‘“”°’°“°° be‘“"~‘°" takes place in N ereis and is especially notable the anterior ntokous part , ' . . . . and the posterior epitokous in the palolo-worm, Eumce 1)27'Zd’l8, whose

portion. (From Korschelt . ‘ ' ' ' d and Heme“ aim W00d_ swarmxngm the south Paeificisoften observe

ward.) as a very unusual phenomenon. ANNELIDA 313

A further (-lassificatioii of tho asexual methods of reproduction in this group may he based upon the degree of preparation which precedes

fiG. 211. J! yriamda fumata with 29 zooids. (From Korscholt and Heider. after Maluquin.)

the division. In the simpler cases the division of the worm takes place between two segments with no special zone of separation having been developed. Following the division regeneration occurs producing a head or tail, or in some cases where a portion of the middle of the worm is cut off from both ends both head and tail are regenerated. As has been pointed out this regeneration after division is known as architomy in contradistinction to paratomy where a. separation zone is developed before division. Such a zone is really the beginning of an early regeneration and may be present in variable degrees all the way from a simple region of constriction to the formation of a well—developed head and sensory apparatus.

Architomy is illustrated in Ctenodrzlus monostylus where without much preparation the worm divides into an anterior and posterior part by constriction and the lacking parts are regenerated. Regeneration begins at once, although the new worms continue to creep about actively in spite of the fact that they cannot for a short while take in food. Architomy may be accomplished by autotomous division of the worm. A number of polychaetes and oligochaetes have such great powers of regeneration that they are enabled to separate without external stimulation. Lumbnculus is an annelid of this type.

Paratomy is illustrated by certain species of Ctenodrzlas also, especially C. serratus, in which the regenerative powers are so great that even single segments may in certain cases produce new worms. Beginning a few segments

Fm 012 Tami”: mulomm with a back of the head, cellular prolrferations pnmaw" ,e;;(.,,e,a,,,,g Dome" and mo takeplacewhichgraduallyformthicken ings on the anterior side of each segment.

' They become the head folds and give rise to the organs of the head region in the new zooids. Thus a chain of zooids is formed asexually. Additions of this sort produce what is functionally an alternation of generations. It is scarcely developed with the regularity that characterizes this process in such forms as hydroids. The original individual was developed from the egg. It produces asexually a chain of zooids which separate from each other and when weather conditions become suitable develop further into individuals which reproduce sexually. Of course second and third asexual generations may be produced in the same manner as the first one.

Echinodermata. Among the echinoderms asexual reproduction is described, but it is certainly of rare occurrence and one has difliculty in distinguishing it from an extreme type of regeneration. Spontaneous division of the arms and in some cases the splitting of the disc have been described in the asteroids, ophiuroids, and in the holothurians. Asteroids have been seen to split and separate, beginning at the gullet, and then

flu. 213. S1/Ilis ramosa showing anterior end of worm and the complex branching. The gut is stippled in the figure. (From Korsehelt and Heidor. after Maclntosh.)

the missing arms and organs of the disc to become gradually regenerated. It is of course well known that where accidental separation of the parts of an echinoderm is brought about extensive regenerations follow and theoretically there is no great difference between this and a natural type of division followed by regeneration. However, the latter is certainly of earlier occurrence.

Pterobranchia. The two genera Cephalodiscus and Rhabdopleura which compose the anomalous Pterobranchia both illustrate asexual reproduction by budding. Their doubtful position showing similarities both to Balanoglossus and to the Bryozoa is nevertheless in line with their type of reproduction. Cephalodiscus occurs as a single individual from the side of which a stalk grows out. This stalk bears a bud as is shown in the well-known figure often copied from Maclntosh. The bud arises from the apex of the ventral stalk and after a certain age breaks off. The animal lives in a gelatinous coenoecium where a large number of free individuals may be found. The animal possesses practically all the important organs found in Balanoglossus and structurally seems related to that form. Its method of asexual reproduction, however, much more strongly resembles that of the Polyzoa. Resemblance to the latter group is rather more striking in Rhabdopleura which lives in a tube and reproduces in a manner similar to that of the stoloniferous Bryozoa. The general anatomy, however, closely resembles that of Cephalodiscus. In Rhabdopleura the individuals derived from the buds remain close together and form small colonies living in branched tubes. They are con— nected by a muscular cord which passes back to join a common stem or stolen. The cord is the narrowed proximal portion of the body. By its contraction the animal is retracted into a stalk. The stolen is developed as a bud from the original zooid,

1«'.c.. 214. ('c1)Iw,lod£scus with but the details of the process are not well §:f:‘lt'i$;:f’)“‘M“°I“t°*h'“'i”"“°‘ll‘ understood. Part of the stolen loses its

formative power and becomes merely a connection between the creeping parts of the colony. The remainder, however, buds, and the buds give rise to branches. In the free-growing part there always appear two buds, an anterior, well—developed one and a posterior much younger.

Tunicata. Among the tunicates asexual reproduction is widespread and occurs as an important means of multiplication in two of the main divisions of this subphylum, the composite ascidians and the salps. The dominant type is budding but the details show various departures from the simple form and even instances of fission are observed within this group. It will perhaps serve our purpose to limit our discussion to the budding in the two cases mentioned, understanding that there are some other minor examples to be met with in this group. The embryology of the ascidians has been described in Part One of this book. The young embryo grows and in many cases begins its process of budding before it has become attached and begun its metamorphosis. It is customary to speak of the individual which is produced from the egg as an oozooid whereas the individuals which are produced from buds are blastozooids. However, the individuals so called may become modified

in various ways and be given other names to indicate functional and morphological differences which later occur, for budding in the tunicates as in some other groups leads to polymorphism and to alternation of generations.

In the compound ascidians there are two main types of budding, palleal and stolonial, differentiated according to the position on the oozooid in which they occur. By palleal budding, which also goes under the name of peribranchial or atrial, or also cntero-epicardial, is meant the growth of a bud as an oesophageal or even intestinal outgrowth; it is thus derived near the atrial wall and under the mantle or pallium. It is common for the buds to be formed as lateral evaginations of the body Wall in symmetrical parts, but usually only one of them develops. Mesoderm and germ cells migrate from the tissues of the mother into the bud which gradually becomes a complete individual. The processes are the same in the asexually produced blastozooid and in the oozooid which are developed from the egg, but the oozooid undergoes a retardation in development and never reaches sexual maturity whereas the blastozooid develops fully although it is to be noted that both may reproduce asexually as well as sexually. The oozooid is short lived. After it has reached the typical form of the ascidian it undergoes retrogression and its tissues are used to build up new individuals. In the composite aseidians although the buds are independent of each other they yet remain enclosed in a common cellulose mantle, and after a number of generations have

‘ ‘~ .-..-.-I!l!‘."'€.;',.7,_,,’

Fm. 215. Budding in Rhabdopleura normanni.

(From a diagram by Korschelt and Heider.)

b. buds of different ranks from a common stolon (st.).

been produced the mass takes on the form of buds united into a very definite system built upon the plan of concentric circles of buds. As the system becomes more and more complicated some of the individuals become crowded out of their proper place and so become the center of a new irregular subdivision of buds. In this way a colony is formed. The place of origin of the buds varies in difierent genera, a fact which gives rise to the marked differences in appearance of different colonies of compound ascidians. Thus we have pyloric budding which is also called oesophageal, epicardo-oesophageal, and epicardo-rectal budding. In this type when the blastozooid buds it is really of compound origin, the new process arising from abdominal and thoracic outgrowths, the abdominal bud being an invagination of the oesophagus and the thoracic formed largely from the epicardium. From a diverticulum of the left portion of the epicardium the nervous system is derived. It is thus endodermal in origin in the blastozooid, although in the oozooid it is derived in the regular manner from the ectoderm. Pyloric budding is described chiefly in the didemnids and the diplosomids. Among the modifications of the general type of budding described for the compound ascidians are the cases spoken of as pseudo-stolonial, occurring especially in the distomid Distaplia and in the polyclinids. Authorities differ in the manner in which they regard the budding of these forms, some tracing it to the formation of a stolon and others regarding it as more nearly related to the type already described. In Distaplia the formation of the “primordial buds” has been described as coming from the epicardium, but Salensky, Julin, Della Valle, and others have described this as a proliferating stolon from the intestine. The first generation of these buds never develops sex products but later generations of buds may reproduce sexually. It may be noted that the epicardium is wrongly named in that it does not participate in the formation of the pericardium and the heart, although it was so described by van Beneden and J ulin. Actually it is the endodermal element of each bud. In the polyclinids the budding of the posterior part of the animal resembles horizontal division, for the caudal part of the larva clongates and constricts, forming thorax, abdomen, and post-abdomen; gonads and heart develop in the latter which subsequently segments to form buds. By some this entire caudal portion is regarded as a stolon.

As an example of stolonial budding reference may be made to Perophora and to Clavellina. In Clavellina, after its attachment, root—like processes are sent out from the base giving it a more firm hold. At least one of these processes becomes the proliferating stolon (stolo prolzfer). The stolon consists of the three germ layers in addition to the cellulose tunic which covers it. From it arise buds as well as side branches. If a particular outgrowth contains an endodermal layer it would develop into a bud. Otherwise it is merely a sterile branch. Buds arise near the growing tip of the stolon, but simultaneously with their development the top continues to grow, so those near the base of the stolon are the more mature. Younger ones sometimes arise as a new generation of buds between the older ones. Stolonial budding takes place only from the oézooid for the blastozooid can reproduce sexually. It should be noted in regard to the formation of colonies that although the usual condition is the production of an entire colony from a single oozooid, yet small colonies sometimes grow so close together that concrescence results and they fuse into a larger colony.

F16. 216 An adult of Clavellma with a stolen and .i young individual budded ofi from it (From Korschelt and Hcider, after Secliger )

In many respects the most interesting case of asexual reproduction in the tunicates is the formation of the Salpa chains from a proliferating stoloii. When investigators first began to study Salpa, solitary individuals were found, and also others which appeared to have a significantly different structure and were connected together like a chain or even a rosette were also occasionally discovered. At length the relation between the two was made out by the poet, Chamisso, who discovered that the solitary individuals can produce the chain salps and that certain individuals of the chains developed sex organs from which the 320 ASEXUAL REPRODUCTION

solitary individuals were again formed. The solitary individual always reproduces asexually but is itself produced from a fertilized egg. From the ventral side of the posterior half of such a solitary individual a bud, which first arose from the pharyngeal endostyle and is therefore in its beginning eiidodermal, grows out. As it enlarges, the mesoderm and ectoderm surround it, and finally it becomes a projection from the tunic as well. It is a direct continuation of the ectoderm and endoderm of the

fiG 217. Nearly adult stages of Dolwlum dentwulatum (From Korschclt and H0|d(‘r, after Neumann.)

cl , cloacu, d c , dorsal horn or cadophore to which buds Will be uttuchcd, st , \CIlU‘dl stolon from whuh buds are produced, t , mil with chorda

parent individual and its parts are likewise related to the organs which develop in its buds derived from it. This is the ventral proliferating stolen and the individual producing it is the oozooid developed from the single egg of the gonozooid.

The history of the ventral stolon differs somewhat in Salpa and Doliolum. Perhaps the latter, as described by Uljanin, is the one more familiar to students. As in the compound ascidians the oozooid produces no sexual organs but a great many buds arise from it in a complicated manner and the chain thus produced exhibits a high degree of polymorphism. Indeed, the oozooid itself degenerates until it becomes nothing but a locomotor organ of the rest of the chain. The buds are produced continuously for a considerable time from the ventral stolon. They do not remain in that part of the body, however, but migrate dorsally to a posterior dorsal horn, or cadophore, which is an outgrowth resembling somewhat a stolen but does not of itself bud. Its epithelial cells become specially suited for the attachment and nourishment of the migrating buds which will presently reach it. As a matter of fact, however, there are more buds in the dorsal cadophore than were produced from the ventral stolon, multiplication in the form of a simple division having taken place during the migration. The cadophore continues its growth and more and more buds are attached to it. They arrange themselves in two lateral rows and one dorsal median row. The lateral blastozooids which are attached by a short stalk are nutritive and respiratory. They are apparently unable to reproduce and are morphologically independent of the rest of the colony. The median blastozooids are spoken of as nurse individuals, or phorozooids. They are rounded and have a stalk on the ventral side. It is this stalk which bears the buds that in their turn are to give rise to the sexual forms. Like the lateral blastozooids, the nurses are non-reproductive. The buds which arise from the stalk of the phorozooids are usually spoken of as protogonozooids. From these the sexual blastozooids, that is, the gonozooids, are produced. They are free swimming, undergo a considerable change in their general structure, develop sex organs, and the cycle is completed, for from the fertilized egg comes the new zooid ready to begin the asexual cycle again. Thus there are two distinct asexual generations, the oozooid and the protogonozooid, and some of the blastozooids have also multiplied by simple division in passing to the dorsal cadophore. These two asexual generations alternate with the one sexual generation. The development of all forms of Salpa is not understood yet and in some cases it is difficult to interpret the forms that have been found. The life cycle of Doliolum, however, illustrates satisfactorily the degree to which asexual reproduction is developed in these forms.

Vertebrate. In forms higher than the tunicates asexual reproduction does not commonly occur. However, the case of the armadillo which is described in detail in the chapter on polyembryony must be mentioned. Here a blastocyst is formed as a result of the cleavage of the egg which produces by budding four individuals. This is, of course, an asexual method of multiplication, and may be of general significance, as is suggested by the production of identical twins in various other forms of mammals.

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1931 Richards: Part One General Embryology 1 Historical Development of Embryology | 2 The Germ-Cell Cycle | 3 Egg and Cleavage Types | 4 Holoblastic Types of Cleavage | 5 Meroblastic Types of Cleavage | 6 Types of Blastulae | 7 Endoderm Formation | 8 Mesoderm Formation | 9 Types of Invertebrate Larvae | 10 Formation of the Mammalian Embryo | 11 Egg and Embryonic Membranes | Part Two Embryological Problems 1 The Origin And Development Of Germ Cells | 2 Germ-Layer Theory | 3 The Recapitulation Theory | 4 Asexual Reproduction | 5 Parthenogenesis | 6 Paedogenesis And Neoteny | 7 Polyembryony | 8 The Determination Problem | 9 Ecological Control Of Invertebrate Larval Types

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