Vertebrate Embryology - A Text-book for Students and Practitioners (1893) 1
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Marshall AM. Vertebrate Embryology: A Text-book for Students and Practitioners. (1893) Elder Smith & Co., London.
|Marshall (1893): 1 Introduction | 2 Amphioxus | 3 Frog | 4 Chick | 5 The Rabbit | 6 Human Embryo | Illustrations|
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- 1 Chapter I. Introduction
- 1.1 General Account of the Development of Animals
- 1.2 Structure of the Egg
- 1.3 Maturation or Ripening of the Egg
- 1.4 Fertilisation of the Egg
- 1.5 The Early Stages of Development of the Embryo
- 1.6 Theory of Fertilisation
- 1.7 Segmentation of the Egg
- 1.8 The Germinal Layers
- 1.9 The General History of Development : the Recapitulation Theory
- 1.10 The Origin of Sex
- 1.11 Bibliography
Chapter I. Introduction
General Account of the Development of Animals
ALL animals may be referred to one or other of two great groups, Protozoa and Metazoa. Of these, the former, or Protozoa, are minute, often microscopic animals, which throughout their whole lives remain single cells. Most Protozoa lead solitary existences, but there are several that give rise to colonies by continuous fission ; in such colonies, however, each member, though organically connected with its neighbours, is physiologically independent of them, and discharges all the great functions of life for itself.
The second group, or Metazoa, includes all remaining animals, from sponges to man. It is characterised by the fact that the adult animal consists, not of a single cell, but of many cells. which are variously modified in different parts to form the digestive, respiratory, nervous, and other organs.
In a Protozoon, such as Amoeba or Paramecium, the entire animal is a single unit or cell, and all the activities of the living organism nutrition, respiration, sensation, &c. have to be carried on within the compass of that single cell.
A Metazoon, on the other hand, such as a jelly-fish, a snail, a beetle, or a frog, is built up of a number of such units or cells, which share the work of life amongst themselves.
In the simpler Metazoa, as sponges, or zoophytes, there are comparatively few kinds of cells, an outer protective and sensory layer, and an inner digestive layer being the most conspicuous.
In the higher Metazoa differentiation is carried much further, the number of kinds of cells is greatly increased, and the differences between them are far more pronounced; so much so, indeed, that in a single organ, as the heart of a rabbit, there may be more kinds of cells, and cells differing more widely from one another both in structure and in function, than are to be found in the entire body of one of the simpler Metazoa, such as a hydra.
In all Metazoa, however, whether high or low in the scale of organisation, a distinction may be drawn between the cells which compose the body of the individual proper, and certain other cells which are concerned, not with the welfare of the individual animal, but with the perpetuation of the species. An adult Metazoon consists, in fact, of two chief kinds of cells, somatic and reproductive, of which the former build up the various tissues and organs of the animal itself; while the latter, i.e. the eggs or germ-cells of the female, and the spermcells of the male, contribute nothing towards the maintenance of the animal itself, but provide for the production in due time of future generations of similar animals.
All Metazoa reproduce by means of eggs ; and these eggs are in all cases component cells of the animals in which they occur. Other modes of reproduction are seen, especially in the lower Metazoa, such as the budding of a sponge or of a hydra, but these only alternate with sexual reproduction, egg-producing individuals always occurring sooner or later in the series.
The distinction between Protozoa and Metazoa may now be stated more fully. Protozoa are animals which begin their existence as single cells, and which remain single cells throughout their whole lives. Metazoa are animals which begin their existence as eggs, i.e. which commence, like Protozoa, as single cells ; but in the course of development become multicellular, the majority of the constituent cells becoming modified to form the various parts of the adult animal, while some become reproductive cells, which contribute nothing towards the welfare of the animal itself, but which provide for the continuance of the species.
The life history of a Metazoon usually shows a more or less marked division into two periods or stages, nutritive and reproductive ; the growth of the individual being completed, or nearly so, before the reproductive phase commences. In Vertebrates, and especially in fish, the two periods usually overlap each other, the reproductive organs attaining maturity before growth of the animal as a whole is completed ; but in some Invertebrates the division becomes a very sharp one. The silkworm-moth and the Ephemera or may-fly afford well-known instances of this ; the greater part of the life cycle being spent as larvse, which feed vigorously and grow rapidly, but are incapable of reproduction ; while the adult insects, the moth or the may-fly, are capable of reproducing, but take no food and live but for a few hours.
Structure of the Egg
The egg is a single nucleated cell. In some Invertebrates, as in Sagitta and in certain insects, the cells from which the eggs arise may be distinguished at a very early stage in the development of the embryo, or even from its actual commencement ; but in most Invertebrates, and in all Vertebrates, the somatic and reproductive cells are at first indistinguishable from one another, and it is not until the embryo has advanced considerably in its development that the reproductive cells can be recognised as such.
The eggs, or germ-cells, may be distributed over a considerable part of the body of the animal, as in the Nemertine worms, in Balanoglossus, and, to a less extent, in Amphioxus ; more usually, and constantly in the higher members of a group, they are restricted to particular organs, the ovaries. In the early stages of development, the reproductive organs usually extend over a greater part of the length of the animal than they do in the adult condition ; in the frog, for example, the ovaries undergo during development an actual shortening or concentration, a considerable part of their length degenerating.
The egg or germ-cell, like any ordinary cell, consists of a cell-body, containing a nucleus, and inclosed within an elastic vitelline membrane. The egg is usually more or less spherical, but may be irregular in shape, or even, as in Hydra, amoeboid. The cell-body consists of protoplasm, in which a more or less pronounced reticular structure is present ; the protoplasm forming a network of firmer strands, the meshes of which are filled with a more fluid substance, in which are contained minute particles or granules in greater or less number. The nucleus, or germinal vesicle as it is often called, is large, sometimes as much as half the diameter of the egg itself: it is usually placed excentrically, and consists of an outer nuclear membrane, enclosing a clear coagulable liquid, the nucleoplasm. Traversing the nucleoplasm is a reticulum, formed of one or more complexly coiled threads of a substance which, from the readiness with which it absorbs colouring matters, is termed chromatin. One or more nucleoli, or germinal spots, are very commonly present as small, deeply-staining spherical bodies : they appear, however, to be non-essential structures, and are in many cases only nodes or local thickenings of the reticulum. It is stated by some investigators that the nuclear membrane is not a continuous structure, but is really a denser and more superficial part of the nuclear reticulum ; and that the nuclear reticulum and the reticulum of the protoplasmic cell-body are directly continuous with each other.
Food-Yolk. The meshes of the protoplasmic cell-body of the egg always contain granules. These vary greatly in number and in size in the eggs of different animals, and constitute a store of nutrient matter, at the expense of which the development of the egg and the formation of the embryo are effected ; they may be spoken of collectively as deutoplasm or foodyolk.
These granules of food-yolk, though always present in greater or less quantity, are to be regarded as accessory rather than as essential parts of the egg. In every egg we must distinguish between (1) the living protoplasm of the egg, out of which the embryo is directly developed, and which may be spoken of as germ-yolk ; and (2) the deutoplasm, or granules of food-yolk, consisting of non-living particles of nutritious matter, imbedded in the meshes of the protoplasm, which do not directly form any part of the embryo, but which indirectly render its development possible, by nourishing the active protoplasm.
The relation between the protoplasm or germ-yolk and the deutoplasm or food-yolk is, in fact, the same as that between the traveller and the sandwiches or other provisions that he carries with him to nourish him during his journey. The sandwiches are non-living ; but during the journey they are gradually consumed, absorbed, and utilised for the nutrition of the living tissues of the traveller.
Food-yolk plays so important a part in the development of animals that it is well to consider its influence in some detail.
The amount of food-yolk present is the main factor in determining the size of the egg ; and the differences in size between the eggs of the cod and of the dogfish, or between those of Amphioxus, the frog, and the hen, depend almost entirely on the fact that the cod's egg contains but little food-yolk, and that of the dogfish a great deal ; and that the egg of Amphioxus is almost devoid of food-yolk, while the frog's egg contains a considerable amount, and the hen's egg an enormous quantity. The size of an egg depends on the amount of food-yolk present in the egg, and not on the size of the animal that produces the egg. A crayfish lays larger eggs than a lobster, although the adult crayfish is not more than a third the length of the lobster ; a cuckoo lays much smaller eggs than other birds of its own size ; and a rabbit is developed from an egg less than a sixteenth the diameter of a frog's egg.
The amount of food-yolk determines the actual size, and the degree of development, which the embryo is able to attain at the expense of the egg itself. If the quantity of nutritive material within the egg is small, then it will be quickly absorbed, and the young animal must hatch early, and consequently of small size and imperfect development. If there is a greater amount of food-yolk present, then a larger proportion of the developmental history can be completed before the time of hatching ; while in cases where the eggs are of great size, owing to great abundance of food-yolk, practically the whole development can be effected at the expense of the egg itself, and the young animal hatches in the form of the parent.
Amphioxus lays eggs which measure only about ^(jth inch in diameter, and the young embryos consequently hatch of very small size and in a very immature condition. The frog lays larger eggs, about y^th inch in diameter, which contain sufficient food-yolk to carry the embryo up to the tadpole stage before hatching ; though the rest of the development, from the tadpole to the frog, must be completed at the expense of food obtained by the tadpole during its free living existence. A hen's egg, on the other hand, is large enough, i.e. contains food enough, to enable the embryo to proceed much further in its development before hatching ; and the young chick leaves the egg with all the essential characters of the parent already established.
Large size of eggs implies diminution in number of the eggs, and hence of the offspring ; and it can well be understood that while some animals derive advantage in the struggle for existence by producing the maximum number of young, to others it is of greater importance that the young on hatching should be of considerable size and strength, and able to begin the world on their own account. In other words, some animals may gain by producing a large number of small eggs, others by producing a smaller number of eggs of larger size, i.e. provided with more food-yolk.
The immediate effect of a large amount of food-yolk is to mechanically retard the processes of development ; the ultimate result is to greatly shorten the time occupied by development. This apparent paradox is readily explained. A small egg, such as that of Amphioxus, starts its development rapidly, and in about eight hours gives rise to a free swimming larva, capable of independent existence, with a digestive cavity and nervous system already present ; while a large egg, like that of the hen, hampered by the great mass of food-yolk by which it is distended, has, in the same time, made but very slight progress.
From this time, however, other considerations begin to tell. Amphioxus has been able to make this rapid start owing to its relative freedom from food-yolk. This freedom now becomes a retarding influence, for the larva, having already exhausted the supply of food originally contained within the egg, must devote much of its energies to hunting for, and to digesting, its food ; and hence its further development will proceed slowly.
The chick embryo, on the other hand, has an abundant supply of food in the egg itself; it has no occasion to spend time in searching for food, but can devote its whole energies to completing its development. Hence, except in the earliest stages, the chick develops far more rapidly than Amphioxus, and attains its adult condition in a much shorter time.
Mammals, and some other forms, present apparent exceptions to the rule stated above with regard to the influence of food-yolk on the course of development. A rabbit is developed from a very small egg, an egg which is, indeed, even smaller than that of Amphioxus; and yet the young rabbit at the time of birth has already attained a considerable size, and has proceeded nearly as far in its development as a chick at the time of hatching. This is rendered possible by a special structure, the placenta, by which the embryo rabbit is supplied throughout its development with food, not from the egg itself, but directly from the blood of the mother.
Food-yolk is differently situated in the eggs of different animals. When only a small amount is present, it is fairly uniformly distributed through the protoplasm of the egg-body. Such eggs, as those of Amphioxus or of the rabbit, are called alecithal.
When food-yolk is more abundant, it usually accumulates towards one pole of the egg, the opposite pole being comparatively free from yolk granules. Such eggs are called telolecithal ; and in them, as in the frog's egg, development commences and proceeds more rapidly at the pole in which there is least foodyolk ; while in cases like the hen's egg, in which food-yolk is extremely abundant and the egg is consequently of great size, the developmental processes may be actually confined to this pole.
In crabs and insects, and other members of the group of Arthropods the food-yolk accumulates towards the centre of the egg, the outermost layer of the egg, round its whole periphery, remaining almost free from yolk granules. In such eggs, which are called centrolecithal, development commences simultaneously over the whole surface ; the central part of the egg, owing to the hampering effect of the food- yolk, not taking part in the developmental processes until a comparatively late stage.
All eggs are devoid of yolk granules during the earlier stages of their formation in the ovary. The yolk granules are usually elaborated in special cells, which form capsules or follicles around the eggs ; and the granules are passed from these follicular cells into the interior of the eggs themselves.
Maturation or Ripening of the Egg
After reaching its full size, and usually at or about the time at which it leaves the ovary, but before the commencement of actual development, the egg undergoes certain changes, which are referred to as maturation or ripening ; and which may be considered as a preparation on the part of the egg for fertilisation by the spermatozoon or male element. The changes in question have been studied most completely in the eggs of Ascaris megalocephala, a thread-worm found living parasitically in the horse ; and in the eggs of certain Echinoderms. In Vertebrates they have not as yet been followed in such detail, but all the more important phases have been seen to occur in the eggs of frogs and other Amphibians, and several of them in the eggs of rabbits and other Vertebrates. The changes appear to be essentially the same in all animals in which they have been observed.
The process of maturation concerns the egg nucleus, or germinal vesicle, almost exclusively ; and the principal stages are as follows. The nucleus, which prior to maturation is of large size, with a well-developed nuclear membrane and reticulum, begins to shrink ; the nuclear membrane becomes wrinkled, so that the surface of the nucleus presents an irregular warty appearance (Fig. 1, A). Part of the nuclear fluid exudes through the nuclear membrane into the substance of the egg ; a great part of the nuclear reticulum disappears, or becomes broken up into isolated globules or nucleoli, a very small part alone remaining as a slender intricately-coiled thread, the nuclear skein.
The nuclear membrane now shrinks still further, and finally disappears completely ; the nuclear fluid and nucleoli become distributed through the substance of the egg, and of the original egg-nucleus all that now remains is the minute nuclear skein (Fig. 1, B).
The nuclear skein, which was at first placed centrally, or more or less excentrically, now moves to the surface of the egg. The skein, previously an irregularly tangled thread, assumes the definite form and arrangement of a nuclear spindle, such as is seen in the nucleus of an epithelial or other cell immediately before division of the cell occurs ; it then divides into two equal parts, one of which remains within the egg, while the other is extruded from it as the first polar body (Fig. 1, C). After a brief pause the half of this nuclear spindle that has remained within the egg again divides into two equal parts, one of which is extruded as the second polar body, while the other remains within the egg, and is known as the female pronucleus (Fig. 1, D). The formation of the female pronucleus, by the separation and extrusion of the two polar bodies, completes the process of maturation.
FIG. 1. Successive stages in the maturation of the egg of the Frog. The eggs are represented as bisected vertically, x 25. (After 0. Schultze.)
A, stage in which the nucleus has commenced to shrink, and the nuclear skein is formed in its centre. Q, stage in which the nuclear skein has moved to the surface of the eggr, just prior to formation of the first polar body. C) stage in which the first polar body lias been formed, by division of the nuclear skein, and extruded. D, stage in which the second polar body has been extruded, and the remaining part of the nuclear skein, or female pronucleus, has retreated from the surface of the egg and is about to unite with the male pronucleus, or head of the spermatozoon.
PB, first polar body. PB', second polar body. UF, female pronucleus. TIG, egg nucleus, or germinal vesicle. T7H, fluid exuded from germinal vesicle. TJM, male pronucleus. Z, vitelline membrane.
Concerning the real nature and significance of the changes described above there has been much discussion, and the matter is not yet thoroughly understood. The points of chief importance appear to be the following.
- The process of maturation concerns the egg alone. It takes place when an egg has reached its full size, irrespective of any influence, direct or indirect, on the part of the male animal ; and it occurs in the same manner whether the egg is going to develop into an embryo or not. Apparent exceptions to this statement are met with in cases such as the lamprey and the frog, in which the first polar body is extruded prior to entrance of the male element or spermatozoon, but the second one during or after that process ; while in some few instances, as in Ascaris according to Van Beneden, both polar bodies are extruded after the entrance of the spermatozoon into the egg. It is probable that in these cases the polar bodies are formed as usual, before the entrance of the spermatozoon, but are not extruded from the egg until after that event.
- The changes in the egg-nucleus that precede or accompany the formation of polar bodies are of two distinct kinds : (i) The preliminary reduction in size of the nucleus, and diffusion of the greater part of its substance through the protoplasm of the egg ; (ii) the division of the small remaining portion of the nucleus into the female pronucleus and the polar bodies. These two processes are clearly of widely different nature.
- Of the former of these processes the following explanation has been offered. The great size of the egg-nucleus distinguishes it from the nucleus of almost all other cells, excepting nerve ganglion cells, and is probably associated with the large size of the egg itself and the nutritive changes necessary for its formation and elaboration. When the egg has attained its full size the nutritive or trophic function of the nucleus is fulfilled, and the portion of the nucleus concerned with these processes becomes merged in the protoplasm of the egg-body.
- In the formation of the polar bodies, the nuclear changes that precede or accompany the process appear to be the same as those which occur during the division, by mitosis, of an ordinary epithelial cell. At each division to form a polar body the chromatin threads of the nucleus appear to be halved precisely ; so that the female pronucleus contains exactly one-fourth of the quantity of chromatin present in the nuclear spindle of the egg-nucleus.
- Authorities differ as to whether any part of the protoplasm of the egg is extruded with the daughter nuclei in the polar bodies or not. The point is of considerable interest, for on the one view the formation of a polar body would be merely the division of the egg cell into two very unequal portions, accompanied by the ordinary phenomena of mitosis ; while on the other view the process would be of a very unusual character, consisting in nuclear division, with extrusion of one of the daughter nuclei. The actual details of the process, and more especially a comparison with corresponding changes that occur in plants, strongly support the former view, that the formation of polar bodies is an act not merely of nuclear division, but of true cell division.
- The polar bodies play no part whatever in the development of the embryo. They persist for some time after their formation, but ultimately disappear completely.
- In the great majority of cases that have been studied, including representatives of almost all the great groups of Metazoa, two polar bodies are formed in succession, as described above. The first polar body, after its separation from the egg, not uncommonly divides into two, giving three polar bodies in all. Weismann and Blochmann have shown that the eggs of certain Entomostraca, and of Aphis, which develop parthenogenetically, i.e. without requiring fertilisation by the male element or spermatozoon, form only one polar body.
- After extrusion of two polar bodies an egg appears to be, as a rule, incapable of developing into an embryo unless and until it is fertilised by a spermatozoon. The rule is not absolute, for at least two exceptions are known : the eggs of the gipsy moth, Liparis dispar, and the eggs of the hive bee from which drones are developed, are stated to extrude two polar bodies, and yet to develop without being fertilised.
The above facts indicate that there is a close, though not in all cases a necessary connection between the formation of polar bodies and the act of fertilisation ; and the further consideration of the matter may well be postponed until the latter process has been described.
Fertilisation of the Egg
With certain exceptions which will be noted further on, an egg before it can commence to develop into an embryo requires to be fertilised.
Fertilisation, or impregnation, consists in the fusion of the male element, or spermatozoon, with the female element, or egg ; or, more strictly speaking, fusion of the nuclei of these two bodies.
The spermatozoa of the male animal present certain points of resemblance with the ova or eggs of the female. The early stages of development of the two are closely similar, or even identical, and at the time of first appearance of the reproductive organs in the embryo it is impossible to say of which sex it will subsequently become. In the developing testis, as in the ovary, the essential elements are spherical cells, distinguished from their neighbours by their larger size, -and usually spoken of as primitive ova, but better termed primary reproductive cells or gonoblasts. In the female these gonoblasts become the permanent ova or eggs, usually directly, but sometimes after fusion with one another. In the male each gonoblast, by repeated division, gives rise to a number of cells which, by elongation, become converted into the spermatozoa. In this formation of spermatozoa a larger or smaller part of the gonoblast may take no share, but remain unaltered as the blastophore, a portion of granular protoplasm, round which the spermatozoa are arranged.
The fully formed spermatozoon is a cell, consisting of a head, often rod-like in shape and composed almost entirely of the nucleus, and a long vibratile tail by which the active movements of the spermatozoon are effected. In being a single nucleated cell, the spermatozoon resembles the egg or female cell, as it does also in being derived from a primary reproductive cell or gonoblast.
In other respects the male and female elements, spermatozoa and ova, differ from each other markedly. The ovum is a large, more or- Jess spherical cell, with little or no power of movement. The spermatozoon is a minute cell, usually of a rod-like shape, and exhibiting active movements ; the spermatozoa of some animals, as the Crustacea, are, however, spherical and motionless. An ovum is very commonly formed by direct conversion from a single primary reproductive cell or gonoblast, while in some cases two or more gonoblasts may fuse to form a single ovum : in the formation of the male elements, on the other hand, a single gonoblast gives rise, not to one spermatozoon, but to a large number ; all the spermatozoa derived from a single gonoblast must be regarded as together equivalent to a single ovum. Finally, it is the ovum, not the spermatozoon, that gives rise directly to the embryo : the cases of parthenogenesis seen, for example, in Entomostraca or in the Aphides, show that under certain conditions an egg can develop into an embryo without any participation of spermatozoa, but under no circumstances can a spermatozoon develop into an embryo.
The act of fertilisation is effected thus. The ripe spermatozoa gain access to the ova, either through being introduced into the genital ducts of the female, or, as commonly occurs in aquatic animals, through being discharged by the male over the eggs as soon as these have been laid by the female. By their active swimming movements the spermatozoa quickly make their way to the eggs, bore their way through the vitelline membranes, and so enter the substance of the eggs themselves. A single spermatozoon is sufficient to fertilise an egg, and it is doubtful whether more than one is ever normally concerned in the process ; indeed, after one spermatozoon has entered an egg, others seem incapable of making their way in. On entering the egg the spermatozoon loses its tail, but its head or nucleus, now spoken of as the male pronucleus, penetrates into the interior of the egg, and makes its way towards the female pronucleus, i.e. the part of the egg nucleus which remains within the egg after extrusion of the polar bodies, for an egg cannot be fertilised until it has ' matured.' The male and female pronuclei rapidly approach each other, meet, and unite to form a single body, the segmentation nucleus.
The formation of the segmentation nucleus, which completes the act of fertilisation, appears to take place in a very regular and orderly manner, though it is probable that the details are not the same in all cases. The amount of chromatin in the two pronuclei, male and female, is precisely equal in many, though apparently not in all cases ; and the arrangement of the chromatin threads during the formation of the segmentation nucleus is a very definite one, so that the male and female threads can be distinguished throughout the whole process.
The Early Stages of Development of the Embryo
After formation of the segmentation nucleus, the development of the embryo commences almost directly. The earliest stages of development consist in repeated division of the egg, whereby it becomes converted from the unicellular condition, which is permanent only in the Protozoa, to the multicellular state characteristic of all higher animals, or Metazoa. To these early processes of development the name segmentation is given.
Segmentation is essentially a process of cell division, in which the segmentation nucleus plays the same part that the nucleus of an ordinary epithelial cell does in the act of division of the cell ; the nucleus dividing first, and then the body of the cell ; and the process being repeated again and again, until from the single cell or ovum a very large number of cells are produced, from which by further division all the component cells of the adult animal are ultimately derived.
Special attention has been paid to the behaviour of the male and female elements of the segmentation nucleus during its first division. In the Nematode genus Leptodera, Nussbaum states that the two pronuclei, male and female, take up a position parallel to the long axis of the egg, which is ovoid in shape, and then fuse together lengthways to form the segmentation nucleus. The first segmentation plane is a longitudinal one, and passes along the axes of the fused pronuclei, so that each of the two cells formed by the first cleft contains one halt of the male pronucleus and one half of the female pronucleus. Inasmuch as all the cells of the body of the adult animal are derived by division from the two primary ones, it follows, as Nussbaum points out, that if this equal division of male and female nuclear elements obtains in the later stages of cell division, each cell of the adult animal will possess a nucleus derived in precisely equal proportions from the father and from the mother.
This suggestion, the bearing of which on theories of heredity is of great interest, has received striking confirmation from the researches of Van Beneden on the eggs of Ascaris. Van Beneden finds that after extrusion of the polar bodies, and entrance of the spermatozoon into the egg, the two pronuclei, male and female, which are precisely equal in all respects, come very close together, but do not fuse directly. Each pronucleus contains at first a single, much convoluted, and varicose thread of chromatin, which soon divides transversely into two, each of which becomes bent into a U-shaped loop. There are thus four loops in all, two male and two female. Each loop now splits longitudinally into two sister threads. A spindle figure, with pole bodies and polar rays at its apices now appears, and the outlines of the pronuclei, previously distinct, disappear. The chromatin loops, of which, owing to the longitudinal splitting, there are now eight, four male and four female, take up a position at the equator of the spindle. The two sister threads of each pair then separate, one moving towards one pole of the spindle, the other towards the opposite pole ; so that at each pole of the spindle there is a group consisting of two male threads and two female threads. Each group then forms a daughter nucleus, by fusion of the threads to form a skein, and the entire egg divides into the two first segmentation cells, or blastomeres.
This equal division of male and female elements in the first segmentation is of great interest in reference to the theory of fertilisation. If it should prove to occur in the later, as well as in the earliest stages of development, then, as pointed out above, it will follow that the nucleus of each cell in the body of an adult animal will contain male and female elements derived from the male and female pronuclei, i.e. from the father and the mother, in precisely equal amounts. In other words, each cell of the adult body may be spoken of as hermaphrodite. If this be true, then the egg, which in its first origin is merely an epithelial eel* must itself be hermaphrodite.
Theory of Fertilisation
The view developed above, that the egg is to be regarded as hermaphrodite, led to the suggestion, by Minot, that the extrusion of polar bodies may be an act by which the egg gets rid of its male elements ; a view adopted by Balfour, who added the further suggestion that after the formation of the polar bodies the part of the egg nucleus remaining within the ovum, i.e. the female pronucleus, is incapable of further development without the addition of the nuclear part of the male element, or male pronucleus ; and that the habit of forming polar bodies has been acquired by ova for the express purpose of preventing parthenogenesis, and of rendering fertilisation indispensable.
This view is exceedingly suggestive, and with slight modifications has been widely adopted. There are, however, very serious objections to it. It does not explain why the formation of polar bodies so closely resembles, or is even identical with the ordinary process of cell division ; nor why it is that in the great majority of cases three-fourths of the chromatin of the egg nucleus are extruded in the polar bodies. But Weismann's discovery that one polar body is extruded from parthenogenetic eggs is alone sufficient to render revision of the theory necessary.
Weismann has himself attempted to get over the difficulty by an elaborate theory which assumes that the two polar bodies are of entirely different nature, and that it is only the second one, the one not usually formed by parthenogenetic eggs, that contains the male elements. The actual mode of formation of the two polar bodies is, however, strongly opposed to the view that there is a fundamental difference between them ; for in all cases that have been carefully observed, the first and second polar bodies are formed in precisely similar manner. But the dis-. co very noted above, that, in the cases of the gipsy moth and the drone bee, eggs that have extruded two polar bodies can still develop parthenogenetically, is fatal to Weismann's theory.
A more profitable line of inquiry is to compare carefully the phenomena of fertilisation in the Metazoa with those of the lowest animals, and with those of plants. This has been done by Biitschli and Giard, and more recently and in great detail by Hartog. Such a comparison shows that it is a very common occurrence for a primary reproductive cell to give rise, by two or more divisions, to a number of cells of which one alone becomes an ovum, capable of developing into an embryo ; while the others serve as accessory organs for the support or nutrition of the ovum, or for facilitating the access of spermatozoa to it ; or else degenerate and disappear. The formation of polar bodies is probably of similar nature, and is to be regarded as an act of true cell division. By two divisions for the first polar body frequently, perhaps generally, divides after separation from the egg the primary reproductive cell, or gonoblast, becomes divided into four cells, i.e. into three small polar bodies and one large ovum. Each of these four cells contains exactly the same amount of nuclear matter, for this is halved at each division ; and the difference between the ovum and the polar bodies is simply that, as regards the protoplasm of the cell body, the division has been an extremely unequal one ; the ovum having appropriated almost the whole of the protoplasm, while the polar bodies possess exceedingly small amounts of this.
The connection between the formation of polar bodies and the process of fertilisation still remains to be explained. Such cases as those of the gipsy moth and the drone bee indicate that this connection is to be regarded rather as a normal than as a necessary one. Rapid cell division is an exhausting process, and Maupas has shown that in the Ciliate Infusoria the act of fission, which is the most frequent mode of reproduction, although it commences and at first proceeds with great rapidity, after a certain number of generations becomes less rapid, then irregular, and finally ceases altogether. To set it going again, a process of rejuvenescence or constitutional invigoration is necessary ; this is effected by conjugation, during which an interchange of nuclear matter is effected between the two individuals concerned in the act.
It seems very possible that the repeated cell division, which takes place in the formation of polar bodies, has a similar exhausting effect on the nucleus of the ovum, rendering a process of rejuvenescence desirable, and in most cases absolutely necessary, before any further division can take place ; this rejuvenescence being effected by conjugation, or fusion, of the nuclei of the spermatozoon and of the ovum.
This view, as Hartog points out, is in accordance with Balfour's theory in so far as it regards the formation of polar bodies as a process the object of which is to prevent parthenogenesis ; but differs from this theory in regarding the polar bodies, not as male elements extruded from an originally hermaphrodite egg, but as cells, the rapid formation of which has reduced the part of the nucleus still remaining in the egg, i.e. the [female pronucleus, to a condition of exhaustion which renders the stimulus of fertilisation necessaiy, or at least highly advantageous, if further cell-division is to take place.
Segmentation of the Egg
The actual details of segmentation vary considerably in different cases, the differences depending chiefly on the relative amount of food-yolk present, and on its distribution within the egg.
The simplest form of segmentation is presented by alecithal eggs, such as those of Amphioxus. It is characterised by the almost geometrical regularity with which the successive divisions occur, and by the fact that the cells, or blastomeres, into which the egg is divided are approximately equal to one another in size. . The first cleft, Fig. 2, n, is a vertical one, and divides the egg into two perfectly similar halves. The second cleft is also vertical, but at right angles to the first one : on its completion the egg is divided into four cells or blastomeres of equal size, Fig. 2, in. The third cleft, Fig. 2, IV, is a horizontal one, and divides each of the four blastomeres of the previous stage into two, of which the lower one is slightly the larger. Two vertical clefts next appear simultaneously, at angles of 45 with the two first clefts : by these the number of the blastomeres is again doubled, giving sixteen in all, Fig. 2, v. Two new horizontal clefts double the number of blastomeres once more ; the stage, with thirty-two blastomeres, being shown in Fig. 2, vi. From this time segmentation continues rapidly, but with less regularity : later stages are shown in Fig. 2, vn and vm.
Fig. 2. Segmentation of the egg of Amphioxus. x 220. (After Hatschek.)
I, the egg before the commencement of development : only one polar body, PB, has been seen, but from analogy with other animals it is probable that there are really two present. II, the ovum in the act of dividing, by a vertical cleft, into two equal blastomeres. Ill, stage with four equal blastomeres. XV, stage with eight blastomeres ; an upper tier of four slightly smaller ones, and a lower tier of four slightly larger ones. V, stage with sixteen blastomeres, in two tiers, each of eight. VI, stage with thirtytwo blastomeres, in four tiers, each of eight : the embryo is represented bisected, to show the segmentation cavity or blastocoel, B. VII, later stage : the blastomeres have increased in number by further division. VIII, blastula stage : bisected to show the blastocoel, B.
Segmentation is said to be complete, or holoblastic, when, as in Amphioxus, the whole egg is divided up at once into blastomeres : it is further distinguished as equal when, as again in Amphioxus, the several blastomeres are from the first approximately equal in size.
In the frog's egg, Fig. 3, segmentation is holoblastic, but unequal. The first two clefts, which, as in Amphioxus, are vertical, divide the egg equally and symmetrically ; but the third, or horizontal cleft, Fig. 3, in, is much nearer the upper than the lower pole, and throughout the later stages of segmentation, Fig. 3, IV and v, there is marked inequality in size between the blastomeres of the upper and lower halves of the egg. Unequal segmentation is due to food yolk, which, in a telolecithal egg like the frog's, is specially accumulated in the lower pole, and retards the developmental processes in this as compared with the upper half of the egg.
An exaggeration of this condition is seen in the hen's egg, in which food-yolk is present in such quantity as to absolutely stop the processes of development in all parts of the egg, except in a small circular patch on the surface, corresponding to the upper pole of the egg of Amphioxus or the frog. To this circular patch, or germinal disc, Fig. 4, BA, segmentation is restricted. Figs. 5 and 6 represent surface views of the germinal disc during the process of segmentation, and show the irregular manner in which the several clefts appear ; while Fig. 7 represents a vertical section of the germinal disc, with the adjacent parts of the yolk, at the close of segmentation. Segmentation, when confined to part of the egg, is spoken of as meroblastic ; and when, as in the hen's egg, it is limited to a circular patch on the surface of the egg it is further distinguished as discoidal.
Fig. 3. Segmentation of the Frog's Egg. The second figure is a surface view, the remaining four figures represent the egg in section, x 20.
I, the ovum just before the completion of the first cleft, by which it is divided into two equal blastonieres. II, stage with eight blastomeres : an upper tier of four small ones, and a lower tier of four much larger ones. Ill, the same stage, with eight blastomeres, in section. IV, V, later stages, showing further increase in the number of the blastomeres, with great inequality in their size. B, segmentation cavity or blastoccel. U , nucleus.
Fig. 4. The Hen's Egg, freshly laid, x g.
BA, germinal disc. SH, egg shell. SM, shell membrane. SV, air chamber. ~WA, white or albumen. WC, chalaza. Y yolk. Z, vitelline membrane.
Figs. 5, 6. Stages in the segmentation of the germinal disc of the Hen's Egg. x 10. (After Coste and Duval.)
Fig. 7. Vertical section of the germinal disc of the Hen's Egg at the close of segmentation, x 25. (After Duval.)
B, segmentation cavity or blastocoel. E, upper layer of blastomeres, or epiblast. M"', nucleus of incompletely formed blastomere. VXi, vacuole in yolk. Y, yolk ZL, lower layer of blastomeres.
Another type of meroblastic segmentation is presented by the centrolecithal eggs of Arthropods. Here, there is no localised germinal disc, but the whole surface of the egg consists of a layer of protoplasm free from yolk-granules, in which segmentation occurs almost simultaneously at all parts ; such a mode of segmentation may be distinguished as superficial.
The principal types of segmentation, described above, may be tabulated as follows :
I. Holoblastic or complete segmentation.
A. Equal : as in the alecithal egg of Amphioxus.
B. Unequal : as in the telolecithal egg of the frog. II. Meroblastic or partial segmentation.
C. Liscoidal : as in the telolecithal egg of the chick.
D. Superficial : as in the centrolecithal eggs of Arthropods.
The Germinal Layers
At the close of segmentation the whole of the egg, or, in cases of meroblastic segmentation, a part only of it, is divided up into cells or blastomeres. These blastomeres very early become arranged in two layers ; an outer layer, the epiblast, which covers the surface of the embryo ; and an inner layer, the hypoblast, which lines a cavity within its interior. Epiblast and hypoblast form the two primary germinal layers of the embryo : the epiblast becomes ultimately the epidermis or outer layer of the skin ; while the hypoblast becomes the epithelial lining of the alimentary canal ; the cavity surrounded by the hypoblast, spoken of as the archenteron, forming the first commencement of the digestive tract. Figs. 8 and 9 represent early larvae of Amphioxus which have reached the stage described.
The details of development of epiblast and hypoblast, and more especially the mode of appearance of the archenteric cavity, are subject to great modifications in different groups of animals, but the essential relations are in all cases as described above.
Between epiblast and hypoblast a third layer of cells, the mesoblast, appears at a later stage, usually derived, directly or indirectly, from the hypoblast. Though appearing after the other two germinal layers, the mesoblast grows very rapidly, and in the higher animals forms a larger part of the embryo than the other two layers together.
The two primary germinal layers, epiblast and hypoblast, occur, and with essentially similar relations, in all groups of Metazoa, from sponges up to mammals. The middle germinal layer, or mesoblast, presents far greater variations, and it is by
FIGS. 8, 9. Vertical and horizontal sections of early larval stages of Amphioxns. x 220. (After Hatschek.)
CE, commencing mesoblastic outgrowth. E, epiblast. Q, archenteron. TT, hypoblast. NF, neural fold. NT, neureuteric canal, leading from neural tube to archenteron. PC, polar mesoblast cell.
no means clear that all the structures spoken of as mesoblastic in the different groups of animals have any real community of origin or relations. In Sponges and Coelenterates a mesoblastic layer cannot be said to exist, but in all other groups of Metazoa it is present.
The three germinal layers together make up the whole of the embryo, and from them all parts of the adult animal are derived : the principal organs and parts to which the layers give origin respectively are as follows.
The epiblast, or outer layer, gives rise to the epidermis, covering the body generally ; and to the various organs derived from the epidermis. Of these, the more important are : the nervous system, both central and peripheral ; the olfactory and auditory epithelium, the retina and lens of the eye, and the other organs of sensation ; the epithelial lining of the mouth and anus ; the pineal and pituitary bodies ; the enamel of the teeth ; the hairs, nails, claws, and other epidermal modifications ; and the epithelial lining of the mammary, sweat, and other glands formed from the skin.
The hypoblast, or inner layer, gives rise to the epithelium lining the alimentary canal and its various diverticula ; including the glands of the oesophagus, stomach, and intestine, the lungs, the bladder, the bile ducts, gall bladder, and pancreatic ducts, the hepatic cells of the liver, and the secreting cells of the pancreas. The notochord also is formed from hypoblast.
FIG. 10. Transverse section through the head of a Chick Embryo at the end of the first day of incubation, showing the relations of the three germinal layers, x 100.
B, cavity of the brain : the origin of the walls of the brain from the epiblast is \vell seen. CH, notochord, arising from the hypoblast. E, epiblast. TT", hypoblast. NA, root of one of the cranial nerves. TP, cavity of the alimentary canal, in the pharyngeal region. RT, blood-vessel. The whole of the part of the figure covered by the lighter shading is mesoblast.
From the mesoblast, or middle layer, are derived all structures lying between the epiblast and hypoblast ; i.e. the connective tissue, muscles, skeleton (except the notochord), bloodvessels, and lymphatics ; and also the peritoneum, and the urinary and reproductive organs.
The General History of Development : the Recapitulation Theory
It is a familiar fact that animals in the earlier stages of their existence differ greatly in form, in structure, and in habits from the adult condition.
In some cases, as, for example, in Amphioxus, the whole history of development is a steady upward progress towards the adult condition, the several organs and parts gradually approximating towards the fully formed state, and each stage bringing the animal, not merely as a whole, but as regards each of its organs and parts, one step nearer to the perfect form.
In the great majority of animals, however, the course of development is not so straightforward. Even in Amphioxus there are features in the early embryonic stages, such as the communication between the neural tube and the digestive cavity, which completely disappear during development, and which have no relation to the adult condition of the animal.
In the higher Vertebrates, far more striking instances occur. In the embryo of a chick or of a mammal the structure and relations of the heart and blood-vessels are for a time those of a fish ; and for the attainment of the adult condition it is necessary, not merely that new structures should appear and new relations be acquired, but that parts once present should actually become obliterated. The frog, again, commences its free existence as a tadpole, which is really a fish, not merely as regards its breathing organs, but in all details of its organisation ; and the change from the tadpole to the frog involves great modification in the shape, size, and relations of almost all its organs, with complete obliteration of parts such as gills and tail, which were essential to the tadpole but are absent from the frog.
It is to cases such as the frog, or as the butterfly, in which the transition from larva to adult is even more extensive and more abrupt, that the term metamorphosis is applied ; cases in which the animal, instead of developing straight towards the adult condition, in place of aiming straight at its goal, deviates from the direct path, spends time and energy in developing and elaborating organs which, though in perfect keeping with its actual mode of existence, yet have no relation to the state it is ultimately to reach, and must indeed be got rid of before that final condition can be attained.
Cases of this kind forcibly illustrate the necessity for some explanation of the facts of development. Much attention has been given to the subject, especially of recent years, and it is now possible to frame a consistent theory which will explain the general history of development in all groups of animals, and which will also be in harmony with the accepted views concerning the mutual relations of these groups.
The doctrine of descent, or of evolution, teaches us that as individual animals arise, not spontaneously, but by direct descent from pre-existing animals, so also is it with species, with families, and with larger groups of animals, and so also has it been for all time ; that as the animals of succeeding generations are related together, so also are those of successive geologic periods; that all animals, living or that ever have lived, are united together by blood relationship of varying nearness or remoteness ; and that every animal now in existence has a pedigree stretching back, not merely for ten or a hundred generations, but through all geologic time since life first commenced on the earth.
The study of development has in its turn revealed to us that each animal bears the mark of its ancestry, and is compelled to discover its parentage in its own development ; that the phases through which an animal passes in its progress from the egg to the adult are no accidental freaks, no mere matters of developmental convenience, but represent more or less closely, in more or less modified manner, the successive ancestral stages through which the present condition has been acquired.
Evolution tells us that each animal has had a pedigree in the past. Embryology reveals to us this ancestry, because eveiy animal in its own development repeats its history, climbs up its own genealogical tree.
This Recapitulation Theory, as it is termed, was obscurely hinted at by the elder Agassiz, and suggested more directly in the writings of Von Baer ; but it was first clearly enunciated by Fritz Miiller in 1863, and has since that date formed the foundation on which the explanation of the facts of embryology is based.
The fact that a frog commences its free existence as a tadpole, i.e. to all intents and purposes as a fish, is a very extraordinary one, but it becomes at once intelligible if we interpret it as meaning that frogs are descended from fish, and that every frog is constrained to repeat or recapitulate its pedigree in the course of its own individual development.
Similarly, the long-tailed condition of the young crab at the time of leaving the egg is to be viewed as an indication of the descent of the short-tailed or brachyurous crustaceans from macrurous ancestors; and the presence of gill clefts in the young stages of chicks or rabbits, which when adult are totally devoid of them, or of teeth in the embryo of the whalebone whale, are in like manner to be regarded as reminiscences of former ancestral conditions, and as indicating that the ancestors of chicks and rabbits breathed by gills, and that the toothless whalebone whales are descended from toothed progenitors.
It is on this fact of Recapitulation that the great value of embryology depends. The study of development acquires a new and striking interest when it is realised that through it we are enabled to obtain knowledge, in many cases unattainable by any other means, of the real or blood relationships between animals and groups of animals.
It is with animals as with men, the only natural classification is a genealogical or phylogenetic one, and the possibility of framing such a classification of animals depends very largely on the success with which we are able to reconstruct their pedigrees from a study of the stages through which they pass in their actual development or ontogeny.
Recapitulation must apply, not merely to the development of an animal as a whole, but to that of each one of its organs and parts : the formation of the ear, for example, as a pit of the skin, must be interpreted as meaning that the ear, like the other organs of sensation, was in its earliest commencement merely a specialised patch of skin.
The theory must also apply to the earliest stages of development equally with the later ones ; and the fact that all Metazoa commence their existence as eggs perhaps the most striking of all embryological facts receives an entirely new significance when we interpret it as a reminiscence of a unicellular ancestry for all Metazoa, and as an indication that all the multicellular animals, or Metazoa, are descended from unicellular Protozoa.
From this point of view the earliest developmental stages of Metazoa deserve special attention, as possibly indicating the actual lines of descent of Metazoa from Protozoa. Segmentation is simply cell-division ; and the main difference between cell division in Protozoa and segmentation of the egg of a Metazoon is that, in the former case, the products of division separate from each other as independent unicellular animals, while in the latter they remain in close contact and become constituent units of one multicellular animal. The several stages of segmentation, Fig. 2, II to vu, may be compared with colonies of Protozoa ; while the blastula stage. Fig. 2, vm, reached at the close of segmentation, bears a striking resemblance to such adult forms as Volvox or Pandorina.
There is, however, another side of the question which must not be overlooked. Although it is undoubtedly true that development is to be regarded as a recapitulation of ancestral phases, and that the embryonic history of an animal presents to us a record of the race history, yet it is also an undoubted fact, recognised by all writers on embryology, that the record so obtained is neither a complete nor a straightforward one.
It is indeed a history, but a history of which entire chapters are lost, while in those that remain many pages are misplaced, and others are so blurred as to be illegible ; words, sentences, or entire paragraphs are omitted, and, worse still, alterations or spurious additions of later date have been freely introduced, and at times so cunningly as to defy detection.
Very slight consideration will show that development cannot in all cases be strictly a recapitulation of ancestral stages. It is well known that closely allied animals may differ markedly in their modes of development, which could not be the case if both recapitulated correctly. The common frog, for example, is at first a tadpole breathing by gills, a stage which is entirely omitted by the little West Indian frog, Hylodes. A crayfish, a lobster, and a prawn are allied animals, yet the}* leave the egg in totally different forms. Some developmental stages, as the pupa condition of insects, or the stage in the development of a tadpole in which the oesophagus is imperforate, cannot possibly be ancestral. Or again, a chick embryo, of say the third day, Fig. 113, is clearly not an animal capable of independent existence, and cannot therefore correctly represent any ancestral condition ; an objection which applies to the earlier developmental histories of many, perhaps of most, animals.
Haeckel long ago urged the necessity of distinguishing, in actual development, between those characters which are really historical and inherited, and those which are acquired or spurious additions to the record. The former he terms palingenetic or ancestral characters, the latter cenogenetic or acquired. The distinction is certainly a true one, but an exceedingly difficult one to draw in practice. The causes which prevent development from being a strict recapitulation of ancestral history, the modes in which these came about, and the influence which they respectively exert, are problems which are as yet only partially solved.
Of these disturbing causes, the most potent and the most widely spread arises from the necessity of supplying the embryo with nutriment. This acts in two ways.
If the amount of nutritive matter within the egg be small, then, as we have already seen, the young animal must hatch early and in a very imperfectly developed condition. In such cases, as in Amphioxus or the frog, there is of necessity a long period of larval life, during which natural selection may act so as to introduce modifications of the ancestral history, spurious additions to the text. Of such ' larval organs,' the long spines that form conspicuous features in the young, free swimming larvae of sea urchins, or of crabs, are good examples.
If, on the other hand, the egg contain within itself a considerable quantity of nutrient matter, then the period of hatching can be postponed until this nutrient matter has been used up. The consequence is that the embryo hatches at a much later stage of its development, and, if the amount of food material is sufficient, may even, as in the case of the chick, leave the egg in the form of the parent. In such cases the earlier developmental phases are often greatly condensed and abbreviated ; and as the embryo does not lead a free existence, and has no need to exert itself to obtain food, it commonly happens that these stages are passed through in a very modified form, the embryo being, as in a three-day chick, in a condition in which it is clearly incapable of independent existence.
The effect of a greater or less amount of food-yolk on the recapitulation of ancestral characters has been summed up by Balfour thus : ' There is a greater chance of the ancestral history being lost in forms which develop in the egg, and of its being masked in those which are hatched as larvae.'
There are a number of other causes, besides food-yolk, which tend to modify the ancestral history as preserved in individual development. The following list gives a brief summary of the more important of these.
Causes tending to falsify the ancestral history ; or to prevent ontogeny from being a true record of phylogeny.
1. The general tendency to condensation of the ancestral history. Except perhaps in the lowest groups of Metazoa, such as sponges, no animal can possibly repeat, in its own development, all the ancestral stages in the history of the race. There is a tendency in all animals towards striking a direct path from the egg to the adult : a tendency best marked in the higher, the more complicated members of a group, i.e. those which have the longest and most tortuous pedigrees.
2. The tendency to the omission of ancestral stages. This has been already noticed as one of the commonest effects of abundance of food-yolk. The omission of the gill-breathing stage in Hy lodes and in all Amniote Vertebrates is a . typical example.
3. The tendency to distortion, either in time or space. All embryologists have noticed the tendency to anticipation, or precocious development, of characters which really belong to a later stage in the pedigree. Many early larvas show it markedly, the explanation in this case being that it is essential for them to possess at the time of hatching all the organs necessary for independent existence.
Anachronisms, or actual reversals of the historical order of development of organs or parts, occur frequently. Thus the joint surfaces of bones acquire their characteristic curvatures before movement of one part on another is effected, and even before the joint cavities are formed.
4. The tendency to the accentuation or undue prolongation of certain stages. This is best seen in cases of abrupt metamorphosis, as of the caterpillar to the butterfly ; or of the pelagic pluteus larva to the sea urchin, slowly crawling on the seabottom ; or of the herbivorous aquatic tadpole to the terrestrial and carnivorous frog. In such cases there is usually a great difference between larva and adult in external form and appearance, in manner of life, and very usually in mode of nutrition ; and a gradual transition is inadmissible, because in the intermediate stages the animal would be adapted neither to the larval nor to the adult conditions ; a gradual conversion of the biting mouth parts of the caterpillar to the sucking proboscis of the moth would inevitably lead to starvation. The difficulty is evaded by retaining the external form and habits of one particular stage for an unduly long period, so that the relation of the animal to its surrounding environment remains unaltered, while, internally, preparations for the later changes are in progress.
5. The tendency to the acquisition of new characters. This has been dealt with already ; it arises from the fact that the larval forms of animals, like the adults, are exposed to the action of natural selection, and so are liable to acquire characters that do not belong to the ancestral history.
Before leaving the subject it is worth while inquiring whether any explanation can be found of recapitulation. A complete answer can certainly not be given at present, but a partial one may, perhaps, be found.
Darwin himself suggested that the clue might be found in the consideration that at whatever age a variation first appears in the parent, it tends to reappear at a corresponding age in the offspring ; but this must be regarded rather as a statement of the fundamental fact of embryology than as an explanation of it.
It is probably safe to assume that animals would not recapitulate unless they were compelled to do so : that there must be some constraining influence at work, forcing them to repeat more or less closely the ancestral stages. It is impossible, for instance, to conceive what advantage it can be to a chick or a rabbit embryo to develop gill clefts which are never used, and which disappear at a slightly later stage ; or how it can benefit a whale, that in its embryonic condition it should possess teeth which never cut the gum, and which are lost before birth.
Moreover, the whole history of development in different animals or groups of animals offers to us, as we have seen, a series of ingenious, determined, varied, but more or less unsuccessful efforts to escape from the necessity of recapitulating, and to substitute for the ancestral process a more direct method.
A further consideration of importance is that recapitulation is not seen in all forms of development, but only in development from the egg. In the several forms of asexual development, of which budding is the most frequent and most familiar, there is no repetition of ancestral phases ; neither is there in cases of regeneration of lost parts, such as the tentacle of a snail, the arm of a starfish, or the tail of a lizard ; in such regeneration it is not a larval tentacle, or arm, or tail that is produced, but an adult one.
The most striking point about the development of the higher animals is that they all alike commence as eggs. Looking more closely at the egg, and the conditions of its development, two facts impress us as of special importance : first, the egg is a single cell, and therefore represents morphologically the Protozoan, or earliest, ancestral stage ; secondly, the egg, before it can develop, must, in the great majority of cases, be fertilised by a spermatozoon, just as the stimulus of fertilisation by the pollen grain is necessary before the ovum of a plant will commence to develop into the plant-embryo.
The advantage of cross-fertilisation in increasing the vigour of the offspring is well known, and in plants devices of the the most varied and even extraordinary kind are adopted to ensure that such cross-fertilisation occurs. The essence of the act of cross-fertilisation consists in combination of the nuclei of two cells, male and female, derived from different individuals. The nature of the process is of such a kind that two individual cells are alone concerned in it ; and it may reasonably be argued that the reason why animals commence their existence as eggs, i.e. as single cells, is because it is in this way alone that the advantage of cross- fertilisation can be secured, an advantage admittedly of the greatest importance, and to secure which natural selection would operate powerfully.
The occurrence of parthenogenesis in certain groups, either occasionally or normally, is not so serious an objection to this view as it appears at first. There are strong reasons for holding that parthenogenetic development is a modified form, derived from the sexual method. Moreover, it is the very essence of the view advanced above, that it does not state that cross-fertilisation is essential to individual development, but merely that it is in the highest degree advantageous to the species ; and hence room is left for the occurrence, exceptionally, of parthenogenetic development.
It may be objected that this is laying too much stress on sexual reproduction, and on the advantage of cross-fertilisation ; but it must not be forgotten that sexual reproduction is the characteristic and essential mode of multiplication among Metazoa ; that it occurs in all Metazoa ; and that when asexual reproduction, as by budding, &c., occurs, this merely alternates with the sexual process, which sooner or later becomes necessary.
If the fundamental importance of sexual reproduction to the welfare of the species be granted, and if it be further admitted that Metazoa are descended from Protozoa, then we see that there is a most powerful influence constraining every animal to commence its life history in the unicellular condition, the only condition in which the advantage of cross-fertilisation can be obtained ; i.e. constraining every animal to begin its development at its earliest ancestral stage, at the very bottom of its genealogical tree.
On this view the actual development of any animal is strictly limited at both ends ; it must commence as an egg, and it must end in the likeness of the parent. The problem of recapitulation becomes thereby greatly narrowed ; all that remains being to explain why the intermediate stages in the actual development should repeat, more or less closely, the intermediate stages of the ancestral history. Although narrowed in this way, the problem still remains one of extreme difficulty, and no final solution can yet be given of it.
It is a consequence of the Theory of Natural Selection that identity of structure involves community of descent ; a given result can only be arrived at through a given sequence of events. A negro and a white man have had common ancestors in the past ; and it is through the long-continued action of selection and environment that the two types have gradually been evolved. You cannot turn a white man into a negro merely by sending him to live in Africa : to create a negro the whole ancestral history would have to be repeated, and it may be that it is for the same reason that the embryo must repeat, or recapitulate, its ancestral history in order to reach the adult goal.
Kleinenberg, in his ' Theory of the Development of Organs by Substitution,' has suggested that each historic stage in the evolution of an organ is necessary as a stimulus to the development of the next succeeding stage, and that the reason for the extraordinary persistence, in embryonic life, of organs which are rudimentary and functionless in the adult, may be that the presence of such organs in the embryo is indispensable as a stimulus to the development of the permanent structures of the adult. Should this theory prove to be well founded, it will afford a ready and welcome explanation of many perplexing facts in the development of animals.
The Origin of Sex
The simplest mode of reproduction is a mere act of fission or cell division, as seen in an Amoeba or in an ordinary epithelial cell. Such a form of reproduction is characteristic of the simpler Protozoa, and of the component tissues of Metazoa. It may concern one individual alone, or may be preceded by the conjugation or fusion of two or more originally separate individuals or cells.
The higher Protozoa, or Infusoria, show considerable advance on this simple method. In Parainecium, or Stylonychia, reproduction is effected, as before, by fission, i.e. by division of the single animal into two separate animals ; and under favourable circumstances this process may be repeated again and again with great rapidity. Sooner or later the rate slackens, and ultimately the process stops altogether ; and it does not recommence until conjugation, usually temporary, has occurred between two individuals, which on the completion of the process begin to divide actively once more. Maupas' researches have shown that this conjugation is absolutely necessary, and that it must not take place between two closely allied individuals, but between ones of different broods.
In Vorticella there is further complication, for the conjugating individuals are in this case unlike ; one being an ordinary large, stalked Vorticella ; the other a small free-swimming individual, of which a number, usually eight, are formed by simultaneous division of a large Vorticella. The conjugation is in this case a permanent one, the small Vorticella fusing completely with the large one ; and the whole process corresponds singularly closely with the sexual reproduction of Metazoa, the small free-swimming Vorticella playing the part of the spermatozoon, while the large fixed one behaves as the ovum. This may be taken as the first definite establishment amongst animals of sexual differentiation, and the two Vorticellre may not inappropriately be spoken of as male and female respectively.
In the colonial Protozoa, such as Volvox, which take the form of hollow balls of cells, certain of the cells become large and stationary, forming the female cells or ova ; these are fertilised by small active male cells, derived from the same or from other colonies ; and then, by division of the fertilised ova, new balls or colonies are formed.
This process is essentially the same as the sexual reproduction of Metazoa, and there can be little doubt that the process has been inherited by the Metazoa from their Protozoan ancestors.
The reason for the occurrence of sexual reproduction in all Metazoa is probably to be found, as suggested above, in the consideration that it is through sexual reproduction alone that the full advantage of cross-fertilisation can be obtained. This view, that sexual reproduction is to be regarded as highly advantageous rather than as absolutely essential to the species, is of great importance, as it leaves room for, and renders intelligible, the occurrence of other and asexual modes of reproduction such as are seen in so many groups of Invertebrates. It also affords a clue to the extraordinary condition of things described in certain of the pelagic Tunicates. Salensky has shown that in Salpa, and to a less marked degree in Pyrosoma, certain of the follicle-cells surrounding the ovum pass into its interior, and take an active part in the formation of the embryo ; so that, although the egg is fertilised in the ordinary manner, the blastomeres resulting from its segmentation only give rise to certain of the component cells of the embryo, and not, as is usually the case, to all of them. This mode of development may be regarded as a combination of the ordinary sexual process with an asexual process similar to that by which the geminules of sponges or the statoblasts of Polyzoa are formed.
List of the more important Books and Memoirs bearing on the Subjects of Chapter I.
Balfour, F. M. : ' Treatise on Comparative Embryology.' Vol. i. chaps, i. ii. iii. ; vol. ii. chap. xiii. 1880-81.
Beneden, E. v. : ' Recherches sur la maturation de 1'oeuf et la fecondation.' Archives de Biologie, iv. 1884.
Beneden, E. v., et Neyt, A. : ' Nouvelles recherches sur la fecondation et la division mitotique chez 1'Ascaride megalocphale.' Bulletin de 1' Academic Eoyale des Sciences de Belgique, 3 e ser., tome xiv. 1887.
Blochmann, F. : ' Ueber die Richtungskorper bei Insekteneiern.' Morpho logischcs Jahrbuch, Bd. xii. 1887, und Bd. xv. 1889.
Boveri, T. : ' Zellenstudien,' Heft i. ii. iii. Jenaische Zeitschrift fiir Natur wissensclmft, 1887, 1880, 1890.
Biitschli, O. : ' Gedanken iiber die morphologische Bedeutung der sogenannten Richtungskorperchen.' Biologisches Centralblatt, iv. 1884.
C'arnoy, J. B. : ' Les globes polaires de 1'Ascaris.' La Cellule, tcme ii. iii. 1887.
Geddes and Thomson : ' The Evolution of Sex.' 1889.
Hartog, M. M. : ' Some Problems of Reproduction.' Quarterly Journal of Microscopical Science, vol. xxxiii. 1891.
Hertwig, 0. : ' Lehrbuch der Entwicklungsgeschichte des Menschen und der Wirbelthiere.' Dritte Auflage. 1890.
' Vergleich der Ei- und Samenbildung bei Nematoden.' Archiv fur mikroskopische Anatomic, Bd. xxxvi. 1890.
Kleinenberg, N. : ' Die Entstehung des Annelids aus der Larve von Lopado rhynchus.' Zeitschrift fiir wissenschaftliche Zoologie, Bd. xliv. 1886.
Marshall, A. Milnes : ' Address to the Biological Section of the British Association.' British Association Report, 1890 ; and Nature, vol. xlii. 1890.
Maupas, E. : ' Recherches experirnentales sur la multiplication des Infusoirescilies.' Archives de Zoologie Exp6rimentale, deuxieme s6rie, tome vi. 1888.
' Le rajeunissement karyogamique chez les Cilies.' Archives de Zoologie Exp6rimentale, deuxieme serie, tome vii. 1889.
Minot, C. S. : ' Theorie der Gonoblasten.' Biologisches Centralblatt, Bd. ii. 1882.
Nussbaum, M. : ' Ueber die Yeranderungen der Geschlechtsproducte bis zur Eifurchung.' Archiv fiir mikroskopische Anatomic, Bd. xxiii. 1884. ' Bildung und Anzahl der Richtungskorper bei Cirripedien.' Zoo logischer Anzeiger, xii. 1889.
Salensky, W. : ' Beitriige zur Embryonal-Entwieklung der Pyrosomen.' Zoo logische Jahrbiicher ; Abtheilung fiir Anatomie und Ontogenie, Bd. iv. u. v. 1890-91.
Schultze, O. : ' Untersuchungen iiber die Reifung und Befruchtung des Amphi bieneies.' Zeitschrift fiir wissenschaftliche Zoologie, Bd. xlv. 1887.
Waldeyer, W. : ' Karyokinesis and its relation to the process of Fertilisation, (translation). Quarterly Journal of Microscopical Science, vol. xxx. 1889.
Weismann, A. : Essays upon Heredity and kindred Biological Problems (translations). 1889 and 1892.
Weisrnann, A., und Ischikawa, C. : ' Ueber die Bildung der Richtungskorper beithierischen Eiern.' Berichte der naturforschenden Gesellschaft zu Freiburg i. Br. Bd. iii. 1887.
Zacharias, O. : ' Neue Untersuchungen iiber die Copulation der Geschlechtsproducte und den Befruchtungsvorgang bei Ascaris megalocephala. 1 Archiv fur mikroskopische Anatomie, Bd. xxx. 1887.
|Marshall (1893): 1 Introduction | 2 Amphioxus | 3 Frog | 4 Chick | 5 The Rabbit | 6 Human Embryo | Illustrations|
Marshall AM. Vertebrate Embryology: A Text-book for Students and Practitioners. (1893) Elder Smith & Co., London.
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