Book - Text-Book of the Embryology of Man and Mammals 1
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Hertwig O. Text-book of the embryology of man and mammals. (1892) Translated 1901 by Mark EL. from 3rd German Edition. S. Sonnenschein, London.
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Description of the Sexual Products
IN most animals, and without exception in all Vertebrates, the development of a new being can take place only when reproductive elements, produced by two sexually different individuals, the egg by the female, and the seminal corpuscle or seminal filament by the male, are at the proper time brought into union as the result of the procreative act.
The egg and the seminal filament are simple, elementary parts or cells, which are produced in special glandular organs, the egg-cells in the ovary of the female, and the semen-cells in the testis of the male. After the beginning of sexual maturity at definite periods, they detach themselves within the sexual organs from their union with the remaining cells of the body, and form, under suitable conditions of development, among which the union of the two sexual cells is the most important, the starting-point for a new organism.
First of all, therefore, we have to acquaint ourselves with the peculiarities of the two kinds of sexual products.
The Egg-Cell
The egg is by far the largest cell of the animal body. At a time when nothing was known of its cell-nature, its separate components were given special names, which remain in use even at the present time. The contents were called egg-yolk, or vitettus ; the cell-nucleus was called vesicula germinativa, or germinative vesicle, discovered by the physiologist PURKINJE \ the nuclear corpuscles, or nucleoli, were called germinative spots, or maculce germinativce (WAGNER) ; and, finally, the cell-membrane was called the yolk-membrane, or membrana vitellina. All these parts vary in not unimportant ways from the ordinary condition of the protoplasm and nucleus of most animal cells.
The vitellus (tigs. 1 and 3 n.d) rarely appears homogeneous, mucilaginous, and translucent, like the protoplasm of most cells; it is ordinarily opaque and coarsely granular. This results from the fact that the egg-cell, during its development in the ovary, stores up in itself nutritive materials, or reserve stuff's. These consist of fat, of albuminous substances, and of mixtures of the two, and are described, according to their form, as larger and smaller yolkspherules, yolk-plates, etc. Later, when the process of development is in progress, they are gradually used up in the growth and for the increase of the embryonic cells. The fundamental substance of the egg, in which the reserve stuffs just now referred to are imbedded, is protoplasm, physiologically the most interesting and important of substances, because in it take place, as we infer from many phenomena, the essential life-processes.
We must therefore distinguish in the yolk, in accordance with the suggestion of VAN BENEDEN, (1) the eggprotoplasm, and (2) the yolk-substance, or deutoplcusm, which is of a chemically different nature, and is stored up in the former. When the deposition of reserve materials takes place to a great degree, the really essential substance, the egg-protoplasm, may become almost entirely obscured by it (figs. 3, 4). The protoplasm then fills up the small interstices between the closely packed yolkglobules, yolk-cakes, or lamellae, as mortar does those between the stones in masonry, and appears in sections only as a delicate network, in the smaller and larger meshes of which lie the yolk-elements. Only at the surface of the egg is the egg-plasm constantly present as a thicker or thinner continuous cortical layer.
The germinative vesicle usually occupies the middle of the egg. It is the largest nuclear structure in the animal body, and its diameter generally increases with the size of the egg.
The germinative vesicle (figs. 1, 2) is separated from the yolk by a firm membrane, which may often be distinctly demonstrated, and which surrounds various included components : nuclear liquid (Kernsaft), nuclear network, and nudeoli. The nuclear liquid is more fluid than the yolk, in the fresh condition usually as clear as water, and when coagulated by the addition of reagents, absorbs only a little or no coloring matter. It is traversed by a netivork of delicate filaments (kn), which attach themselves to the nuclear membrane. In this network are enclosed nudeoli, or germinative spots (&/), small, for the most part spherical, homogeneous, lustrous structures, which consist of a substance akin'to protoplasm nuclear substance or nuclein. Nuclein is distinguishable from protoplasm in addition to certain other chemical reactions especially by the fact that it absorbs with great avidity pigments such as carmine, hsematoxylin, aniline, etc., on account of which it has also received from FLEMMING the name chromatin.
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Fig. 1. Immature egg from the ovary of an Echinoderm. The large germinative vesicle shows a germinative dot, or nucleolus, in a network of filaments, the nuclear network.
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The number of the nudeoli in the germinative vesicles of different animals is highly variable, but it is tolerably constant for each species; sometimes there is only a single micleolus present (fig. 1), sometimes there are several or even very many of them (fig. 2kf). Accordingly one may with AUERBACH distinguish uninucleolar, plurinucleolar, and multinucleolar germinative vesicles.
At their surfaces eggs are surrounded by protective envelopes, the number and condition of which are exceedingly variable throughout the animal kingdom as well as among Vertebrates. It is best to divide them, as LUDWIG has done, according to their method of origin, into two groups, into the primary and the secondary eggmembranes. Primary egg-membranes are such as have been produced either by the egg itself or by the follicular cells within the ovary and the egg-follicle. Those produced by the yolk of the egg are called vitelline membrane ; those formed by the follicular epithelium, chorion. All which take their origin outside of the ovary, as a result of secretions on the part of the wall of the oviduct, are to be designated ns secondary egg-membranes.
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Fig. 2. Germinative vesicle of a Frog's egg that is still small and immature. It shows very numerous mostly peripheral germinative spots (kf), in a fine nuclear network (kit), m, Nuclear membrane.
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In their details the eggs of the various species of animals differ from each other in a high degree, so that they must really be considered as the most characteristic for the species of all the kinds of animal cells. Their size, which is due to a greater or less accumulation of deutoplasm, varies so extensively that in some species the egg-cells can be only barely recognised as minute dots, whereas in others they attain the considerable dimensions of a Hen's egg, or even of an Ostrich's egg. The form is usually globular, more rarely oval or cylindrical. Other variations arise from the method in which protoplasm and deutoplasm are constituted and distributed within the limits of the egg ; there are in addition the differences of the finer structure of the germinative vesicle and the great variability of the egg-membranes.
Some of these conditions are of great significance from their influence on the manner of subsequent development. They have been employed as a basis for a classification of the various kinds of eggs.
It is most expedient to divide eggs into two chief groups, into simple and into compound eggs, the first of which is divisible into several sub-groups.
Simple Eggs
Simple eggs are such as are developed in an ovary out of a single germinal cell, The eggs of all the Vertebrates and most of the Invertebrates belong to this group.
In this chief group there occur, according to the manner in which protoplasm and deutoplasm are distributed within the egg, three modifications, which are of very great importance in the determination ofthejirst processes of development.
In the simplest case the deutoplasm, which ordinarily is present only to a limited amount in the correspondingly small egg, is more or less uniformly distributed in the protoplasm (fig. 1). In other cases there has arisen out of this original condition, in conjunction with an increase in the bulk of the yolk-material, an inequality in the distribution of the two egg-substances previously distinguished. The egg-plasma has accumulated in greater abundance at certain regions of the egg -territory, and the deutoplasma at other regions. Consequently, a contrast has arisen between portions of the egg-cell which are richer, and those which are poorer, in protoplasm. A further accentuation of this contrast exercises an extraordinarily broad and profound influence on the first processes of development, which take place in the egg after fertilisation. That is to say, the changes, which further on are embraced under the process of cleavage, make their appearance only at the region of the egg which is richer in protoplasm, whereas the region which is more voluminous and richer in deutoplasm remains apparently quite unaltered, and is not divided up into cells. By this means the contrast, which was already present in the unsegmented egg, becomes during development disproportionately greater and more obvious. The one part undergoes changes, is divided into cells, and out of these produces the individual organs ; the other part remains more or less unaltered, and is gradually employed as nutritive material. Following the example of KEICHERT, the part of the yolk which is richer in protoplasm, and to which the developmental processes remain confined, has been designated formative yolk, and the other nutritive yolk.
The unequal distribution of formative yolk (vitettus formativus] and of nutritive yolk (vitettus nutritivus) within the egg is accomplished in two different ways.
In the one case (fig. 3) the formative yolk is accumulated at one pole of the egg as &jlat germ-disc (k.sch). Inasmuch as its specific gravity is less than that of the nutritive yolk (n.d) collected at the opposite pole, it is always directed upward, and it spreads itself out on the yolk just like a drop of oil on water. In this case, therefore, the egg has undergone a polar differentiation ; when at rest it must always assume a definite position, owing to the unequal weight of the two poles. The dissimilar poles are distinguished : the upper, lighter pole, with the germ-disc, as the animal (A.P); the lender, heavier and richer in yolk, as the vegetative pole (V.P}. The polar differentiation of eggs is often encountered in Vertebrates, and is especially prominent in the classes of Bony Fishes, Eeptiles, and Birds.
In the second case (fig. 4) the formative yolk (b.d) is accumulated over the whole surface of the egg, and surrounds the centrally placed nutritive yolk (n.d) as a uniformly thick, finely granular cortical
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Fig. 3. Diagram of an egg with the nutritive yolk in a polar position. The formative yolk constitutes at the animal pole (A.P) a germ-disc (k.scJi), in which the germinative vesicle (it. 6) is enclosed. The nutritive yolk (/i.cZ) fills the rest of the egg up to the vegetative pole (V.P).
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Fig. 4. Diagram of an egg with the nutritive yolk in the centre. The germinative vesicle (k.V) occupies the middle of the nutritive yolk (n.d), which is enveloped in a mantle of formative yolk (b.d~).
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layer. The egg exhibits central differentiation, and therefore does not assume a constant position when at rest. As in the former case the yolk was polar in position, so here it is central. Such a condition is never encountered in Vertebrates, but it is characteristic of Arthropods.
In order to distinguish the three modifications, BALFOUR has made use of the expressions alecithal, telolecithal, and centrolecithal. He calls those eggs alecithal in which the deutoplasm, in small amount, is uniformly distributed through the protoplasm ; telolecithal, those in which it is accumulated at the vegetative pole ; centrolecithal, those in which the accumulation of deutoplasm has taken place at the centre, In what follows, we shall speak of (1) eggs with uniformly distributed yolk, (2) eggs with polar deutoplasm, and (3) eggs with central deittoplasm.
It is now expedient to illustrate what has just been said by typical examples, and for this purpose the eggs of Mammals, Amphibia, Birds, and Arthropods have been selected. We shall also frequently recur to these in the presentation of the subsequent phases of development.
The egg of Mammals and of Man is exceedingly small, since it measures on the average only 0*2 mm. in diameter. It is for this reason that it was not discovered until the present century in 1827, by CARL ERNST VON BAER. Previously the much larger GRAAFIAN follicle of the ovary, in which the smaller true egg is enclosed, had been erroneously taken for the latter. The Mammalian egg (fig. 5) consists principally of a finely granular protoplasmic substance, which contains dark, fat-like spherules and granules (deutoplasm), and which is turbid and opaque in proportion to the amount of these. The germinative vesicle (k.fy contains a large germinative dot (&/), located, together with a few smaller accessory dots, in a nuclear network (k.n). The egg-membrane is called zona pellucida (z.p}, because it surrounds the yolk as a relatively thick and clear layer. It is a primary membrane, for it is formed within the GRAAFIAN follicle, by the follicnlar cells. Under high magnification the zona pellucida (z.p) appears radially striate, since it is traversed by numerous porecanals, into which, as long as the egg remains in the GRAAFIAN follicle, very fine projections of the follicular cells (f.z) penetrate. These fuse with the egg-plasm, and are probably concerned in the nutrition and growth of the contents of the egg. (RETZIUS.)
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Fig. 5. Egg from a Rabbit's follicle which was 2 mm. in diameter, after WALDEYER. It is surrounded by the zona pelhicida (z.p), on which there rest at one place follicular cells (/..:). The yolk contains deutoplasmic granules (d). In the germinative vesicle (k.b) the nuclear network (A'.n) is especially marked, and contains a large germinative dot (k.f).
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The human ovum is wonderfully like the egg of Mammals in size, in the condition of its contents, and the nature of its membranes. However, it always can be distinguished by means of special, though trifling, characteristics, as the careful investigations of NAGEL have shown. Whereas in the Rabbit lustrous, fat-like spherules render the yolk cloudy, the human ovum retains its transparency during all stages of development, so that one may recognise most accurately all its structural details, even on the living object. The yolk is divided into two layers. The inner layer contains principally deutoplasm, which produces in this case, contrary to most of the Mammals, only a slight cloudiness ; it consists in part of feebly lustrous, in part of highly refractive fragments, some coarser, some finer; but it is not possible to recognise the mutual boundaries of the individual components, as is the case in other Mammals and lower animals, where one distinguishes with great ease granules and distinct drops. The outer layer or peripheral zone of the yolk is more finely granular and still more transparent than the central part, and contains the germinative vesicle with a large germinative dot, in which NAGEL was able to observe amoeboid motions. The zona pellucicla is remarkably broad; it is striate, and is separated from the yolk by a narrow (perivitelline) space. There are two or three layers of follicular cells attached to the periphery of the egg when it is set free from the GRAAFIAN follicle. The long diameters of these cells are arranged in a radial direction around the egg, as is general in Mammals, and it is due to this circumstance that they have received the name corona radiate^ introduced by BISCHOFF. The human egg without the follicular epithelium measures, on the average, 0'17 mm. in diameter.
The eggs of many Worms, Molluscs, Echinoderms, and Co3lenterates agree with the Mammalian egg in their size, and in the method in which protoplasm and deutoplasm are uniformly distributed through the egg.
The eggs of Amphibia, which were cited as the second example, form a transition from simple eggs, with uniform distribution of yolk-material, to eggs with distinctly expressed and externally recognisable polar differentiation. Already these have deposited in themselves a large amount of deutoplasm, and have thereby acquired a very considerable size. The Frog's egg, for example, is stuffed full of closely compacted, fatty-looking yolk-lumps (Dotterschollen) and yolk-plates. The egg protoplasm is in part distributed as a network between the little yolk-plates ; in part it forms a thin cortical layer at the surface of the egg. Upon closer examination, however, the beginning of a polar differentiation is most distinctly recognisable even here. It manifests itself in this way : at one pole, which at the same time appears black on account of a deposit of superficial pigment, the yolk-plates are smaller and enveloped in more abundant egg-plasm ; and also, nrobably as a consequence of this, slight differences in specific gravity are distinguishable between the pigmented and the unpigmented, or the animal and the vegetative, halves of the egg.
The germinative vesicle (fig. 2) lies in the middle of the immature egg, is exceedingly large, even visible to the naked eye, and multinucleolar, inasmuch as there are a hundred or more large germinative dots (kf) distributed immediately under the nuclear membrane.
The envelopes exhibit, in comparison with the Mammalian egg, an increase in number, for to the zona pellucida (zona radiata), which is produced in the follicle, there is subsequently added still another, a secondary envelope. This is a thick, viscid, gelatinous layer, which is secreted by the wall of the oviduct, and which becomes swollen in water.
The polar differentiation, taken, as it were, in the very process of developing in the case of the Amphibia, is found sharply expressed in our third example, the Bird 's egg.
In order to form a correct picture of the condition of the egg-cell in the case of the Hen, or of any i-.b i-.sch other bird, we must seek it while still in the ovary, at the moment when it has finished its growth, and is ready to be set free from the follicle. It is then ascertained that only the spheroidal yolk, the socalled yellow of the egg, which in itself is an enormously large cell (ficr. 6a), is developed in the botryoidal vitelline membrane, the rupture of which is followed by an extrusion of the soft pulpy contents. By careful examination one will discover upon the latter a small white spot, the germinative disc^/r.scA), or discus proligerus, also called scar or cicatricula.. It has a diameter of about 3 or 4 mm., and consists of formative yolk, a finely granular protoplasm with small yolkspherules, which alone is involved in the process of cleavage. In the flattened germinative disc is also found the germinative vesicle, fig. 6a (k.b) and fig. 6b (x), which is likewise somewhat flattened and lenticular.
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Fig 6a (yo1 ^ f * he Hen \ 1A &' J taken from the ovary, k.sch, Germma OVary. It is enclosed in a thin but tive disc ; k.b, genninative vesicle ;=' IT n IT i / 7 7\ j.1 w-d, white yolk; g.d. yellow yolk tolerably firm pellicle (d.h), the d . A , J ^telliBe membrane.
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The remaining chief mass of the egg-cell is nutritive yolk, which is composed of numberless yolk-spherules united by slight traces of egg-plasm, as though by a cement. Information concerning its finer structure is to be gained from thin sections through the hardened egg, which should be cut perpendicularly to the germinative disc. According to differences in staining and in elementary composition, there are now to be distinguished the ivkite and the yellow nutritive yolk (fig. 6a).
The white yolk (iv.d) is present in the egg-cell only in a small quantity ; it forms a thin layer over the whole surface, the white yolk-rind ; secondly, it is accumulated in somewhat greater quantity under the germinative vesicle, for which it at the same time forms a bed or cushion (PANDER'S nucleus) ; and, thirdly, from this region it
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Fig. 6b. Section of the germ-disc of a mature ovarian Hen's egg still enclosed in the capsule, after BALFOUR.
, Connective-tissue capaiile of the egg ; 5, epithelium of the capsule, on the inside of which lies the vitelline membrane reposing iipon the egg ; c, granular substance of the germinative disc ; w.y, white yolk, which passes imperceptibly into the finely granular substance of the disc ; x, germinative vesicle enclosed in a distinct membrane, but shrivelled up ; y, space originally occupied by the germinative vesicle, biit made empty by its shrivelling up.
penetrates in the form of a mortar-pestle into the very centre of the yellow yolk, where it terminates in a knob-like swelling (latebra, PURKINJE). Upon boiling the egg, it is less coagulated, and remains softer than the yellow yolk. In the coagulated condition the latter discloses upon sections a lamellated condition, in that it consists of smaller and larger spherical shells, which envelope the latebra.
The two kinds of yolk also differ from each other in respect to the condition of their elementary particles. The yellow yolk consists of soft plastic spherules (fig. 7 A) from 25 to 100 \L in diameter, which acquire a punctate appearance from the presence of numerous exceedingly minute granules. The elements of the white yolk are for the most part smaller (fig. 7 B), and likewise spherical, but contain one or several large highly refractive granules.
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Fig. 7. Yolk-elements from the Fowl's egg, after BALFOUR. A, Yellow yolk ; B, white yolk.
At the boundary between the two kinds of yolk there are present spherules which effect a transition between them. +++++++++++++++++++++++++++++++++++++++++
The freshly laid Hen's egg (fig. 8) 'has a different appearance from that of such an ovarian egg. This results from the fact that there is deposited around the yolk, when it detaches itself from the ovary and is taken up by the oviduct, several secondary envelopes derived from the wall of the oviduct, viz., the white of the egg, or the albumen, the shell-membrane, and the calcareous shell. Each of these parts is formed in a special region of the Hen's oviduct. The latter is divided into four regions : (1) A narrow ciliated initial part, into which the liberated egg is received, and where it is fertilised by the spermatozoa already accumulated there ; (2) a
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Fig. 8. Diagrammatic longitudinal section of an unincubated Hen's egg, after ALLEN THOMSON. (Somewhat altered.) b.l. Germ-disc ; iv.y. white yolk, which consists of a central flask-shaped mass and a number of concentric layers surrounding the yellow yolk (y.y.) ', v.t. vitelline membrane ; x. a somewhat fluid albuminous layer, which immediately envelopes the yolk ; w. albumen composed of alternating layers of more and less fluid portions ; ch.l. chalazse ; a.ch. air chamber at the blunt end of the egg simply a space between the two layers of the shell-membrane ; i.s.iii. inner, s.m. outer layer of the shell-membrane ; s. shell.
+++++++++++++++++++++++++++++++++++++++++ glandular region, covered with longitudinal furrows, from which the albumen is secreted and spread around the yolk in a thick layer ; (3) a somewhat enlarged part, covered with small villi, the cells of which secrete calcareous salts, and thus cause the formation of the shell ; (4) a short narrower region, through which the egg passes rapidly, and without undergoing any further change, when being deposited.
The envelopes furnished in succession by the oviduct have the following composition : The white of the egg, or albumen (iu), is a mixture of several materials: according to chemical analyses, it contains 12% albumen, 1*5% fat and other extractive materials, 0*5% salts (potassic chloride, sodic chloride, sulphates, and phosphates), and 86% water. It surrounds the yolk in several layers of varying consistency. There is a layer quite closely investing the latter, which is firmer and especially noteworthy because it is prolonged into two peculiar spirally twisted cords, the chalazce, (ck.l), which consist of a very compact albuminous substance, and which make their way through the albumen to the blunt and to the pointed poles of the egg.
The albumen is enclosed by the thin but firm shell-membrane (s.m) (membrana testae), which is composed of felted fibres. It may be separated into two lamellae an outer, which is thicker and firmer, and an inner, which is thinner and smooth. Soon after the egg is laid the two layers separate from each other at the blunt pole, and enclose between them a space filled with air (a.c/i], the so-called air-chamber, which continues to increase in size during incubation, and is of importance for the respiration of the developing Chick.
Finally, the shell, or testa (s), is in close contact with the shellmembrane; it consists of an organic matrix (2%), in which 98% calcareous salts are deposited. It is porous, being traversed by small canals, through which the atmospheric air may gain entrance to the egg. The porosity of the calcareous shell is an absolute necessity for the normal development of the egg, since the vital processes in the protoplasm can take place only when there is a constant supply of oxygen. If the porosity of the shell be destroyed, either by soaking it in oil or closing its pores with varnish, the death of the incubated egg ensues in a very short time.
Compound Eggs
Compound eggs are found only in a few subdivisions of the invertebrated animals, as in the Cestocles, Trematodes, etc. ; they are noteworthy in this respect, that they are produced by the union of numerous cells, which are formed in two different glands of the sexual apparatus of the female, in the germariurn and in the vitellarium. In the germarium is developed the egg-cell in the restricted sense. This is always very small, and consists almost exclusively of egg-plasm. When this cell at its maturity is set free from its surroundings and comes into the sexual outlets, it is obliged to pass the opening of the vitellarium', here there are associated with it a number of yolk-cells, which, owing to deposition of reserve material in the protoplasm, appear turbid and coarsely granular, and which constitute the dower that is given by the maternal organism to the developing germ 011 its way. Thereupon the whole is enclosed in one or several secondary egg-membranes, and now constitutes the compound egg, in which, however, the developmental processes manifest themselves exclusively on the simple germ cell ; it is that alone which is fertilised and segments, while the yolk-cells gradually degenerate and are employed as nutritive material. Thus in this case also, upon closer examination, the general law, that the descendent organism takes its origin from a single cell of the maternal body, suffers no exception.
The Seminal Filaments
In contrast with eggs, which are the largest cells of the animal body, the sperm-cells or sperm-filaments (spermatozoa) are the smallest elementary parts ; they are accumulated in great multitudes in the seminal fluid of the male, but can be recognised in it only by the aid of high magnification, being, for the most part, slender motile filaments. Inasmuch as every cell consists of at least two parts, namely, nucleus and protoplasm, we must look for these parts in this case also. We shall take for description the spermatozoa of Man.
In Man the seminal filaments (fig. 9) are about 0'05 mm. long. One may distinguish as head (&) a short but thick region, which marks the anterior end, as tail a long thread-like appendage (s), and between the two a so-called middle piece (m}.
The head (&) has the form of an oval plate, which is slightly excavated on both surfaces, and is somewhat thinner toward the anterior end. Seen from the side () it presents a certain resemblance to a flattened pear. Chemically considered, it consists of nuclear substance (nuclein or chromatin), as microchemical reactions show. To the head is united, by means of a short part called the middle piece (m), the long thread-like appendage (s), which is composed of protoplasm, and is best compared to a flagellum, because, like the latter, it executes peculiar serpentine motions in virtue of its contractile properties. By means of these motions the spermatozoon moves forwards in the seminal fluid with considerable velocity.
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Fig. 9. Mature spermatozoa of Man, seen in two different positions. Each consists of a head (), a middle piece (;;i), and tail (s).
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The spermatozoa have often been designated and it seems to us with entire justice as ciliate, or still better as flagellate, cells.
The spermatozoa of the remaining Vertebrates have a similar structure to that of Man ; on the whole, the diversity of form which is encountered in the comparative study of the egg-cell in the animal kingdom is wanting here.
That spermatozoa are in reality metamorphosed cells cannot be more clearly demonstrated than by their development. According to the extended observations of LA VALETTE and others, each spermatozoon is formed from a single seminal cell or spermatid, and, to be more precise, the head is formed from the nucleus, the contractile filament from the protoplasm.
The metamorphoses which take place in the development have been investigated with the greatest detail by FLEMMING and HERMANN in the case of Salamandra maculata, the spermatozoa of which are characterised by their very great size. The individual spermatozoon here consists of : (1) a very long head, which has the form of a finely pointed skewer, and takes up stains with avidity ; (2) a short cylindrical middle piece, which differs from the first part in chemical properties also ; (3) the motile caudal filament, which in the Salamander exhibits the additional peculiarity that it is provided with a contractile undulating membrane. Of these three regions the skewer-like head, and probably also the middle piece, arise from the nucleus of the spermatid, whereas the contractile filament is differentiated out of the protoplasm. In the development of the head the nucleus of the seminal cell is seen to become more and more elongated (fig. 10 A, B); at first it takes the form of a pear (fig. 10 A k) ; then it grows out into an elongated cone (fig. 10 B &), the base of which serves as the point of attachment for the middle piece (mst). The cone becomes elongated and narrowed into a rod (fig. 11 A, B}, which is finally converted into the characteristic form of a skewer. With this elongation of the nucleus the chromatic network becomes more and more dense, and at last assumes a quite compact and homogeneous condition, as in the mature spermatozoon. The fundament (Anlage) of the middle piece (figs. 10, 11, A, B, mst) makes its appearance early when the nucleus begins to elongate at that end of the nucleus which was called its base, in the form of a small oval body, which at first takes up stains like the head, but afterwards loses this property. Its first appearance demands still further elucidation.
Why are the male sexual cells so small and thread-like, and so differently constituted from the eggs ?
The dissimilarity between the male and the female sexual cells is explained by the fact that a division of labor has arisen between the two, inasmuch as they have adapted themselves to different missions.
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Fig. 10 A and B. Initial stages of the metamorphosis of the seminal cell into the seminal filament, after HERMANN.
A, Seminal cell with pear-shaped nucleus ; S, seminal cell with cone-shaped nucleus ; sz, seminal cell ; k, nucleus with chromatin network, and nucleoli (n) ', mst, body out of which the middle piece is developed r, ring-like structure, \\ Inch is in contact with th middle piece, and is claimed to have relation to the formation of the spiral membrane of the filament ; f, caudal appendage of the seminal filament.
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Fig. 11 A and B, Two terminal stages in the metamorphosis of the seminal cell into the seminal filament, after FLEMMING.
Ic, Nucleus, which has become elongated to form the head of the spermatozoon ; mst, its middle piece ; /, its caudal filament.
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The female cell has assumed the function of supplying the substances which are necessary for that nutrition and growth of the cell protoplasm which a rapid accomplishment of the process of development demands. It has therefore, while in the ovary, stored up in itself yolk-substance, reserve material, for the future ; and consequently has become large and incapable of motion. But inasmuch as it is necessary for the accomplishment of a process of development that union with a second cell from another individual should take place, and since non-motile bodies cannot unite, therefore the male element has been suitably modified to meet this second requirement,
For the purpose of locomotion .and in order to make possible the union with the non-motile egg-cell, it has become metamorphosed into a contractile filament, and has rid itself completely of all substances, as, for example, yolk-material, which would interfere with this principal requirement. At the same time it has assumed the form best adapted for passing through the envelopes with which, as a means of protection, the egg is surrounded, and for penetrating the yolk.
The conditions especially in the vegetable kingdom confirm the accuracy of this interpretation. There are plants of the lowest forms in which the two copulating sexual cells are entirely alike, both being small and motile ; and there are other related species in which a gradual differentiation is brought about by the fact that one of the cells becomes richer in yolk and incapable of motion, while the other becomes smaller and more active. From this it is evident that the stationary egg must now be sought out by the migratory cell.
A few physiological statements may be in place in this connection. In comparison with other cells of the animal body, and especially in comparison with the eggs, the seminal filaments are characterised by greater duration of life and power of resistance, a fact which is frequently of importance for the success of fertilisation. The mature spermatozoa, after they are set free from their connection with other cells, remain for months in the testes and vasa deferentia without losing their fertilising power. They also appear to remain active for a long time after having been introduced into the sexual passages of the female, perhaps for several weeks in the case of Man. For some animals this is demonstrable to a certainty. For example, it is known that the semen of Bats remains alive in the uterus of the female during the whole winter ; and in the case of the Fowl it is known that fertilised eggs can be laid up to the eighteenth clay after the removal of the Cock.
In the presence of external influences semen shows itself to be much more resistent than the egg-cell, which is easily injured or killed. For example, when semen is frozen and then thawed out, the motion of the seminal filaments comes back again. Many salts, if they are employed not too strong, have no deleterious influence. Narcotics in strong concentration, and when employed for a long time, make the filaments motionless, without immediately killing them, because after removal of the injurious substance they can be revived.
Very weak alkaline solutions stimulate the motions of seminal filaments ; on the contrary, acids, even when they are very dilute, produce death. Accordingly the motion becomes more lively in all animal fluids of alkaline reaction, whereas in acid solutions it soon dies out.
History
The discovery that egg and seminal filament are simple cells is of far-reaching import for the comprehension of the whole process of development. In order to appreciate this to its full extent, it will be necessary to make a digression into the historical field. Such a digression will acquaint us with some fundamental transformations, which have affected our conception of the essentials of developmental processes.
In the last century, and even in the beginning of the present, ideas about the nature of the sexual products were very indistinct. The most distinguished anatomists and physiologists were of opinion that eggs agreed in their structure in every particular with the grown-up organism, and therefore that they possessed from the beginning the same organs in the same position and connection as the latter, only in an extraordinarily diminutive condition. But inasmuch as it was not possible, with the microscopes of the time, actually to see and demonstrate in the eggs at the beginning of their development the assumed organs, recourse was had to the hypothesis that the separate parts, such as nervous system, glands, bones, etc., must be present, not only in a very diminutive, but also in a transparent condition.
In order to make the process more intelligible, the origin of the blossoms of plants from their buds was cited as an illustrative example. Just as already in a small bud all the parts of the flower, such as stamens and coloured petals, are enveloped by the green and still unopened sepals, just as the parts grow in concealment and then suddenly expand into a blossom, so also in the development of animals it was thought that the already present but small and transparent parts grow, gradually expand, and become discernible. The doctrine which has just been outlined was consequently called the Theory of unfolding, or evolution. However, a more appropriate designation for it is the one introduced during recent decennia -preformation theory. For the characteristic feature of this doctrine is, that at no instant of development is there anything new formed, but rather that every part is present from the beginning, or is preformed, and consequently that the very essence of development the becoming is denied. "There is no such thing as becoming is the way it is expressed in the " Elements of Physiology " by HALLER. " No part in the animal body was formed before another ; all were created at the same time."
As the necessary consequence of a rigid adherence to the preformation theory, it follows, and indeed was formulated by LEIBNITZ, HALLER, and others, that in any germ the germs of all subsequent offspring must be established or included, since the animal species are developed from one another in uninterrupted sequence. In the extension of this box-within-box doctrine (Einschachtelunyslehre) its expounders went so far as to compute how many human germs at the least were concentrated in the ovary of mother Eve, and thereby arrived at the number 200,000 millions.
The evolution theory offered a point of attack for a scientific feud, inasmuch as every individual among the higher organisms is developed by means of the cooperation of two separate sexes. When, therefore, the seminal filament os well as the animal egg became known, there soon arose the actively discussed question, whether the egg or the seminal filament was the preformed germ. Deceunium after decennium the antagonistic camps of the ovists and of the anvmalculists stood opposed to each other. Those who followed the latter thought they saw, with the aid of the magnifying glasses of the limes, the spermatozoa of man actually provided with a head, arms, and legs. The animalculists recognised in the egg only a suitable nutritive soil, as it were, which was necessary to the growth of the spermatozoon.
In the face of such doctrines there dawned a new period for Embryology, when in 1759 CASPAR FRIEDEICH WOLFF in his doctor's dissertation opposed the dogma of the evolution theory, and, casting aside preformation, laid down the scientific principle that what one could not recognise by means of his senses was certainly not present preformed in the germ. At the beginning, so he maintained, the germ is nothing else than an unorganised material eliminated from the sexual organs of the parent, which gradually becomes organised, but only during the process of development, in consequence of fertilisation. According to WOLFF, the separate organs of the body differentiate themselves one after another out of the hitherto undifferentiated germinal material. In individual cases he endeavoured, even at this time, to determine more exactly, by means of observations, the nature of the process. Thus C. F. WOLFF was the founder of the doctrine of epigenesis, which, through the discoveries of the present century, has proved to be the right one.*
WOLFF'S doctrine of unorganised germinal matter has been compelled since then to give way to more profound knowledge, thanks to the improved optical aids of recent times, and to. the establishment of the cell -theory by SCHLEIDEN and SCHWANX. A 'better insight into the elementary composition of animals and plants was now acquired, and especially into the finer structure of the sexual products, the egg-cell and the seminal filament.
So far as regards the egg-cell, a series of important works began with PUEKINJE'S investigation of the Hen's egg in 1825, in which the germinative vesicle was described for the first time. This was soon (1827) followed by C. E. V. BABE'S celebrated discovery of the Mammalian egg, which had been hunted for, but always without success. Extensive and comparative investigations into the structure of the egg in the animal kingdom were published in 1836 by R. WAGNEE, who also discovered at the same time in the germinative vesicle the germinative dot (macula germinativa).
With the establishment of the cell-theory there naturally arose the question as to how far the egg was in its structure to be regarded as a cell, a question which was for years answered in widely different ways, and which even now from time to time is brought up for discussion in an altered form. Even at that time SCHWANN, albeit with a certain reservation, expressed it as his opinion that the egg was a cell, and the germinative vesicle its nucleus; but others, his cotemporaries (BISCHOFF and others), regarded the germinative vesicle as a cell, and the yolk as a mass of enveloping substance. A unanimity of views in this matter was brought about only after the general conception of " cell " had received in Histology a more precise definition. This was due especially to more accurate knowledge of the processes of cell-formation gained through the works of NAGELI, KOLLIKER, REMAK, LEYDIG, and others.
- Historical presentations of the theory of evolution and the theory of epigenesis, which are worth the reading, have been given by A. KIECHHOFF in his interesting paper, " CASPAE FEIEDEICH WOLFF. Sein Leben und seine Bedeutung fur die Lehre von der organischen Entwicklung." Jenalsche Zeitsehrift fur Medic hi und Naturwissenschaft, Bd. IV., Leipzig, 1868 ; and by W. His, " Die Theorien der geschlechtlichen Zeugung." Archiv fiir Anthropologie, Bd. IV. u, V.
The interpretation of eggs with separate formative and nutritive yolk, and with partial cleavage, occasioned especial difficulty. Two antagonistic views in this matter have existed for a long time. According to one view, eggs with polar nutritive yolk (the eggs of Reptiles, Birds, etc.) are compound structures, which cannot be designated as simple cells. Only the formative yolk, together with the germinative vesicle, is comparable with the Mammalian egg; the nutritive yolk, on the contrary, is something new, superposed upon the cell from without, a product of the follicular epithelium. The spherules of the white yolk are explained as uninuclear and multinuclear yolk-cells. The formative and nutritive yolk together are comparable with the entire contents of the GRAAFIAN vesicle of Mammals. H. MECKEL, ALLEN THOMSON, ECKER, STRICKER, His, and others, have expressed themselves in favour of this view with slight modifications in the details.
According to the opposite view of LEUCKART, KOLLIKER, GEGENBAUR, HAECKEL, VAN BENEDEN, BALFOUR, and others, the Bird's egg is just as truly a simple cell as the egg of a Mammal, and the comparison with a GRAAFIAN follicle is to be rejected. The yolk never contains enclosed cells, but only nutritive components. As KOLLIKER, especially in opposition to His, has shown, the white-yolk spherules contain no structures comparable with genuine cell-nuclei ; and therefore cannot be interpreted as cells. As GEGENBAUR already in 1861 sharply formulated it : " The eggs of Vertebrates with partial cleavage are on that account essentially no more compound structures than those of the remaining Vertebrates; they are nothing else than enormous cells peculiarly modified for special purposes, but which never surrender this their real character." There would be no change in this interpretation, even if it should prove to be that the yolk was formed in part from the follicular epithelium, and was set free from the latter as a sort of secretion. In that event we should have to do with a special method of nutrition of the egg, the cell-nature of which cannot on that account be called in question.
Various components of the yolk have received special names. REICHERT first distinguished as formative yolk the finely granular mass, which, in the Bird's egg, contains the germinative vesicle, and forms the germ-disc, because it alone undergoes the process of cleavage, and produces the embryo. The other chief mass of the egg he called nutritive yolk, because it does not break up into cells, and because subsequently, enclosed in a yolk-sac, it is consumed as nutritive material. Afterwards His introduced for these the names chief germ and accessory germ (Haiipt- und Neberikeim).
Whereas the nomenclature of REICHERT and His is applicable only to eggs with polar arrangement of nutritive yolk, VAN BENEDEN (1870) has undertaken the division of the substance of the egg from a more general standpoint. He distinguishes between the protoplasmic matrix of the egg, in which, as in every cell in general, the vital processes take place, and the reserve and nutritive materials, which are stored up in the protoplasm in the form of granules, plates, and balls, and which he designates as rleutoplasm. Every egg possesses both components, only in different proportions, in varied forms and distribution. BALFOUR has selected this latter condition as a basis for division ; and has consequently made the three groups of alccitbal, telolecithal, and cent rolecithal eggs, for which I have selected the designation eggs with little or uniformly distributed yolk, eggs with polar, and eggs with central yolk.
In recent times investigation has been directed to the finer structure of the germinative vesicle, in which KLEINENBEEG (1872) was the first to observe a special protoplasmic nuclear trestle (Kerngeriisf) or nuclear network, which since then has been shown by numerous researches to be a constant structure. In the case of the germinative dot I have myself designated two chemically and morphologically distinguishable substances as nuclein and paranuclein, the investigations concerning the importance and the role of which in the develop, ment of the egg are not yet concluded.
The history of the spermatozoa begins with the year 1677. A student in Leyden, HAMM, in the microscopic examination of semen, saw the briskly moving bodies, and communicated his observation to his teacher, the celebrated microscopist LEEUWENHOECK, who instituted more accurate investigations, and published them in several papers, which soon attracted general attention. The sensation caused was all the greater because LEEUWENHOECK declared the seminal filaments to be the preexisting germs of animals, and maintained that at fertilisation they penetrated into the egg-cell and grew up in it. Thus arose the school of animalculists.
After the refutation of the preformation theory, it was thought that no importance was to be ascribed to the seminal filaments in fertilisation, it being held that it was the seminal fluid that fertilised. Even during the first four decennia of the present century, the seminal filaments were almost universally held to be independent parasitic creatures (spermatozoa) comparable with the Infusoria. Even in JOH. MULLER'S " Physiology" (1833-40) occurs this statement : " Whether the semen-animalcules are parasitic animals, or animated elements of the animals in which they occur, cannot for the present be answered with certainty."
The settlement of the question was accomplished by comparative histological investigations of the semen in the animal kingdom, and by physiological experiment.
In two essays " Beitrage zur Kenntniss der Geschlechtsverhaltnisse und der Samenfliissigkeit wirbelloser Thiere," and " Bilduug der Samenfiiden in Bliischen " -K6LLIKEE showed that in many animals, e.g., in the Polyps, the semen consists of filaments only, the fluid being entirely absent ; and that in addition the filaments are developed in cells, and consequently are themselves elementary parts of animals. PtEiCHERT discovered the same to be true in Nematodes. By means of physiological experiment it was recognised that seminal fluid with immature and motionless filaments, and likewise mature but filtered semen, did not fertilise. This was decisive for the view that the seminal filaments are the active part in fertilisation, and that the fluid, which is added thereto in the case of the higher animals under complicated sexual conditions, " can be regarded only as a menstruum for the seminal bodies which is of subordinate physiological significance."
Since then our knowledge (1) of the finer structure, and (2) of the development of the seminal filaments, has made further advances. So far as regards the first point, we have learned, especially through the works of LA VALETTE and SCHWEIGGER-SEIDEL, to distinguish between head, middle piece, and tail, and to know their different chemical and physical properties. The view expressed by KOLLIKER, that ordinarily the seminal filaments were the metamorphosed and elongated nuclei of the seminal cells, underwent a modification. According to the researches of LA VALETTE, only the head of the seminal filament arises from the nucleus, the tail, on the contrary, from the protoplasm of the spermatid. Finally FLEMMING- brought forward convincing proof that it is only the chromatin of the nucleus that is metamorphosed into the head of the seminal filament. Important investigations concerning the development of the seminal filaments in various animals have recently been made by VAN BENEDEN ET JULIN, PLATNER, HERMANN, and others.
Summary
The most important results of this chapter may be briefly summarised as follows:
1. Male and female sexual products are simple cells.
2. The seminal filaments are comparable to flagellate cells. They are usually composed of three portions, head, middle piece, and contractile filament.
3. The seminal filament is developed out of a single cell, the spermatid; the head, and probably also the middle piece, from, the nucleus ; the contractile filament from the protoplasm.
4. The egg-cell consists of egg-plasm and yolk-particles, which are reserve material (deutoplasm), imbedded in it.
5. The quantity and distribution of the deutoplasm in the egg-cell is subject to great variation, and exercises the greatest influence on the course of the first processes of development.
(a) The deutoplasm is small in amount, and uniformly distributed in the egg-plasm.
(b) The deutoplasm is present in greater quantity, and, in consequence of unequal distribution, is more densely accumulated either at one pole of the egg or inits middle. (Polar and central deutoplasm.)
(c) In eggs with polar deutoplasm (eggs with polar differentiation) the pole with more abundant deutoplasmic contents is designated as the vegetative, the opposite one as the animal pole.
(d) In the case of eggs with polar differentiation, the more abundant protoplasm of the animal pole may be sharply differentiated as germ-disc (formative yolk) from the portion which is richer in deutoplasm (nutritive yolk). The developmental processes take place only in the formative yolk, while the nutritive yolk remains 011 the whole passive,
6. Eggs may be divided into several groups and sub-groups according to their development from cells of the ovary alone, or from cells of the ovarium and vitellarium, MS well as according to the distribution of the deutoplasm, as exhibited in the following scheme :
I. Simple eggs. (Development from cells of the ovary.)
A. Eggs with little deutoplasm uniformly distributed through the egg (alecithal*). (Amphioxus, Mammals, Man.)
B. Eggs with abundant and unequally distributed deutoplasm.
(1) Eggs with polar differentiation (telolecithal), with deutoplasm having a polar position, with animal and vegetative poles. (Cyclostomes, Amphibia.)
(2) Eggs with polar differentiation, which are distinguished from the preceding sub-group by the fact that with them there has been effected a still sharper segregation into formative yolk (germ-disc) and nutritive yolk into a part which is active during development and a part that is passive. (Eggs having polar differentiation with a germ-disc. Fishes, Reptiles, Birds.)
(3) Eggs having central differentiation with central deuto plasm (centrolecithal) and superficially distributed formative yolk (blastema, Keimhaut}. (Arthropods.) II. Compound eggs. (Double origin from cells of the ovarium and vitellarium.)
Literature
Baer, C. E. von. De ovi marnmaliurn et homiuis genesi epistola. Lipsiae 1S27. Beneden, Ed. van. Eecherches sur la composition et la signification de 1'cBiif. Mem. cour. de 1'Acad. roy. Sci. de Belgique. T. XXXIV. 1870. BischofF. Entwicklungsgeschichte des Kanincheneies. 1842. Flemming. Zellsubstanz, Kern- mid Zelltheilung. Leipzig 1882. Frommann, K. Das Ei. Kealencyclopadie der gesammten Heilkunde. 2.
Auflage. Gegenbaur, C. Ueber den Ban und die Entwicklung der Wirbelthiereier rnit partieller Dottertheilung. Archiv f. Anat. und Physiol. 1861. Guldberg. Beitrag zur Kenntniss der Eierstockseier bei Echidna. Sitzungsb.
d. Jena. Gesellsch. (1885), p. 113. Hensen. Die Physiologic der Zeugung. Hermann's Handbuch der Physiologie. Bd. VI. Theil II. Leipzig 1881.
- The translator has been accustomed for several years to use the word homolecithal instead of alecithal, heterolecithal being employed as a coordinate term to embrace telolecithal and centrolecithal eggs.
Hertwig, Oscar. Beitrage zur Kenntniss der Bildung. Befruchtung uncl Tbeilung cles thierischen Eies. Morphol. Jahrb. Bde I. III. IV. 1875 -77, -78. His, W. TJntersuchnngen iiber die erste Anlage des Wirbelthierleibes. I.
Die Entwicklung des Hiihnchens im Ei. Leipzig 1868. Kleinenberg. Hydra. Leipzig 1872. Leuckart, R. Article " Zeugung 1! in Wagner's Handworterbuch der Physio logie, Bd. IV. 1853. Leyd.ig, Fr. Beitrage zur Kenntniss des thierischen Eies im unbefruchteten Zustand. Zool. Jahrbiicher. Abth. f. Anat. Bd. III. (1888), p. 287. Ludwig, Hubert. Ueber die Eibilduug im Thierreiche. Wurzburg 1874. Nagel, W. Das menschliche Ei. Archiv f. mikr. Anat. Bd. XXXI. 1888. Purkinje. Symbolae ad ovi avium historiam ante incubationem. Lipsiae 1825. Retzius. Zur Kenntniss vom Bau des Eierstockeies und des Graafschen Follikels. Hygiea Festband 2. 1889. Schwann. Mikroskopische Untersuchungen iiber die Uebereinstimmung in der Structur und dem Wachsthum der Thiere und Pflanzen. 1839. Engl.
transl. by H. Smith. London 1847. Thomson, Allen. Article " Ovum " in Todd's Cyclopasdia of Anatomy and Physiology. Vol. X. 1859.
Wagner, R, Prodromus hist, generationis. Lipsiae 1836. Waldeyer, W. Eierstock und Ei. Leipzig 1870. Waldeyer, W. Eierstock u. Nebeneierstock. Strieker's Handbuch der Lehre v. den Geweben. 1871. Engl. transl. New York 1872.
Benecke, B. Ueber Reifung und Befruchtung des Eies bei den Fledermausen.
Zool. Anzeiger (1879), p. 304. Beneden, Ed. van, et Charles Julin. La spermatogenese chez 1'Ascaride megalocephale. Bull, de 1'Acad. roy. Sci. de Belgique. T. VII. (1884), p. 312. Eimer. Ueber die Fortpflanzung der Fledermause. Zool. Anzeiger (1879), p. 425. Engelmann. Ueber die Flimmerbewegung. Jena. Zeitschr. f. Med. und Naturwiss. Bd. IV. (1868), p. 321. Flemming, W. Beitrage zur Kenntniss der Zelle und ihrer Lebenserscheuv ungen. II. Theil. Archiv f. mikr. Auat. Bd. XVIII. 1880. Flemming, W. Weitere Beobachtungen iiber die Entwicklung der Spermato somen bei Salamandra maculosa. Archiv f. mikr. Anat. Bd. XXXI.
1888. Hermann. Beitrage zur Histologie des Hodens. Archiv f . mikr. Anat. Bd.
XXXIV. 1889. Hertwig, Oscar, und Richard Hertwig. Ueber den Befruchtuugs- und Theilungsvorgang des thierischen Eies unter dem Einfluss ausserer Agen tien. 1887. Kdlliker. Physiologische Studien iiber die Samenflussigkeit. Zeitschr. f.
wiss. Zoologie. Bd. VII. (1856), p. 201. Kdlliker. Beitrage zur Kenntniss der Geschlechtsverhaltnisse und der Samenfliissigkeit wirbelloser Thiere, etc. Berlin 1841.
Kolliker. Die Bildung der Samenfiiden in Bliischen. Denkschr. d. Schweizcr.
Gesellsch. f. Naturwiss. Bd. VIII. 1847. Nussbaum, M. Ueber die Veranderuugen der Geschlechtsproducte bis zur Eifurchung. Archiv f. mikr. Anat. Bd. XXIII. 1884. Reichert. Beitrag zur Entwickelungsgeschichte der Samenkorperchen bei den Nematoden. Miiller's Archiv. 1847. Schweigger-Seidel. Ueber die Samenkorperchen und ihre Entwicklung.
Archiv. f. mikr. Anat. Bd. I. 1865. Valette St. George, von La. Article " Hoden," Strieker's Handbuch der Lehre von den Geweben. Engl. trans. New York 1872. Valette St. George, von La. Spermatologische Beitrage. Archiv f . mikr.
Anat. Bde. 25, 27, 28. 1885, -86. Waldeyer. Bau und Entwicklung der Samenfiiden. Anat. Anzeiger (1887), p. 345. (Full list of the literature on Spermatozoa.)
Text-Book of the Embryology of Man and Mammals: Description of the Sexual Products | The Phenomena of the Maturation of the Egg and the Process of Fertilisation | The Process of Cleavage | General Discussion of the Principles of Development | The Development of the Two Primary Germ-Layers | The Development of the Two Middle Germ-Layers | History of the Germ-Layer Theory | Development of the Primitive Segments | Development of Connective Substance and Blood | Establishment of the External Form of the Body | The Foetal Membranes of Reptiles and Birds | The Foetal Membranes of Mammals | The Foetal Membranes of Man | The Organs of the Inner Germ-Layer - The Alimentary Tube with its Appended Organs | The Organs of the Outer Germ-Layer | The Development of the Nervous System | The Development of the Sensory Organs | The Development of the Skin and its Accessory Organs | The Organs of the Intermediate Layer or Mesenchyme | The Development of the Blood-vessel System | The Development of the Skeleton
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