Book - An Introduction to the Study of Embryology 2

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Haddon An Introduction to the Study of Embryology. (1887) P. Blakiston, Son & Co., Philadelphia.
Haddon 1887: Chapter I. Maturation and Fertilisation of Ovum | Chapter II. Segmentation and Gastrulation | Chapter III. Formation of Mesoblast | Chapter IV. General Formation of the Body and Appendages | Chapter V. Organs from Epiblast | Chapter VI Organs from Hypoblast | Chapter VII. Organs from Mesoblast | Chapter VIII. General Considerations | Appendix A | Appendix B

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Chapter II. Segmentation and Gastrulation

On the cessation of the various phenomena related above, the oosperm becomes spherical in contour, and its nucleus reappears as a clear rounded vesicle, enclosing a distinct round nucleolus.

This nucleus is properly termed the “ segmentation nucleus as it differs fundamentally from the original nucleus of the unfertilised ovarian ovum. The name “ germinal vesicle,- which is commonly applied to the nucleus of ova, is open to the objection that it is used indiscriminately for the nucleus both before and after fertilisation ; it will here be confined to the former condition.

A - Invertebrates. - Typical or Alecithal Segmentation. - In order to gain a clear comprehension of the segmentation of the oosperm, it will be advisable to take as an example a form in which the process is not obscured by secondary details. The early stages of segmentation can be readily studied in the eggs of most Freshwater Molluscs. The fSTudibranch Mollusc Elysia viridis also serves very well for this purpose ; and the following account refers to the segmentation, as seen in the living egg, of that form.

After a resting-stage, the nucleus divides into two, the nucleolus having immediately before similarly divided (fig. 12, A-c), and each new nucleus travels to an opposite pole of the oosperm. Whilst this is taking place, the nuclei, as such, disappear ; being apparently replaced by two stars, some of the rays of which meet one another in the middle line. These polar stars , as they are termed, are composed of the radially arranged granular protoplasm of the cell. The polar stars then entirely separate, and the oosperm usually becomes distinctly amoeboid, especially at its upper pole. A shallow groove makes its appearance on the surface of the oosperm midway between the two nuclear foci. The groove rapidly deepens (fig. 12, D, e), and eventually divides the oosperm into two distinct spherical halves, which immediately afterwards become appressed together. The nucleus has by this time reappeared as a clear spot in the centre of each polar star ; the rays of the latter disappear and the chromatin collects to form a new nucleolus (fig. 12, F, g). Each segmentation sphere now passes through a short resting-stage.

The application of staining reagents reveals the nature of the changes undergone by the nucleus in segmenting oosperms. In all cases the nucleus is transformed into a spindle-like arrangement of delicate fibres, termed the “nuclear spindle- round the apices of which the cell-protoplasm radiates as the above-mentioned polar stars. The chromatin aggregates at the centre of each fibre, and divides transversly into two, each moiety travelling along its own

Fig. 12. - Early Stages of Segmentation of Elysia viridis (drawn from the living egg).

A. oosperm in state of rest after the extrusion of the polar cells ; B. the nucleolus alone has divided ; C. the nucleus is dividing ; D. the nucleus, as such, has disappeared, first segmentation furrow appears ; E. later stage ; F. oosperm divided into two distinct segmentation spheres, the clear nuclear space in the centre of the aster of granules is growing larger ; G. resting-stage of appressed two spheres ; H. I. similar stages in the production of four spheres ; K. formation of eight-celled stage.

fibre towards the nearest apex of the nuclear spindle (fig. 1 3, d -f). The fibres between the two receding masses of chromatin thin out, and eventually disappear. Finally, the nuclear substance segregates into an ordinary resting nucleus and nucleolus.

The Behaviour of the Nucleus in Cell-Division. - The behaviour of the nucleus during cell- division has received a great deal of attention within the last few years ; and as it is a subject of considerable importance, it will be advisable to give a brief account of the process.

The nucleus of a typical tissue-cell consists of a rounded vesicle containing a nuclear matrix, which is termed “ achromatin- as it is only lightly coloured on the application of staining reagents. The achromatin is permeated by a delicate network or reticulum of a denser substance, the “nucleoplasm- or “chromatin- which also forms the delicate wall of the vesicle. This network readily stains deeply, and the intersections of the fibres usually give a dotted appearance to the nucleus. When the cell is in a resting condition, the chromatin is, as a rule, concentrated either into several rounded bodies, or more frequently into a single mass, the nucleolus ; but this is usually, if not always, connected with the wall of the nucleus by delicate strands of chromatin.

During the process of division in such a nucleus as that just described, the con

A-H. karyokinesis of a tissue-cell. A. nuclear reticulum in its ordinary state.

B. preparing for division ; the contour is less defined, and the fibres thicker and less intricate. C. wreath-stage ; the chromatin is arranged in a complicated looping round the equator of the achromatin spindle. D. monaster-stage ; the chromatin now appears as centripetal equatorial V -s, each of which should be represented as double. B. a migration of the half of each chromatin loop towards opposite poles of the spindle. F. di aster-stage ; the chromatin forms a star round each pole of a spindle, each aster being connected by strands of achromatin.

G. daughter wreath-stage ; the newly formed nuclei are passing through their retrogressive development, which is completed in the resting-stage, H.

d-f. karyokinesis of an egg-cell, showing the smaller amount of chromatin than in the tissue-cell. The stages d. e. f. correspond to D. E. F. respectively. The polar star at the end of the spindle is composed of protoplasm granules of the cell itself, and must not be mistaken for the diaster (F). The coarse lines represent the chromatin, the fine lines the achromatin, and the dotted lines cellgranules [chiefly modified from Flemming]. X-Z. direct nuclear division in the cells of the embryonic integument of the European Scorpion [ after Blochmann ].

tour becomes less defined, owing to the disappearance of its membrane ; the very fine close network appears looser in texture and coarser in fibre ; and a contorted looped rosette or wreath of chromatin is eventually formed (fig. 13, A-c). The peripheral loops fracture, leaving a star-like group of Y-shaped bars of chromatin (aster or single star), the angles of which point towards the centre. By this time the achromatin has been transformed into a nuclear spindle, and the chromatin wreath and single aster lie at right angles to it in its equatorial plane (c, d). Each bent chromatin bar next divides longitudinally (the division is not shown in the figure), and the loops, instead of pointing inwards, become directed, some towards one pole of the long axis of the nucleus, and some towards the other, forming a double star or diaster. It is important to remember that each half of every longitudinally split chromatin bar of the single aster travels towards an opposite pole of the spindle to form the daughter-stars (fig. 13, e, f). Thus, the chromatin of every new nucleus is not formed by the simple partition of the parent nuclear network, but by an actual longitudinal splitting of the chromatin fibre itself, by this means ensuring a perfectly equable division, while the preliminary breaking up of the network into bent bars facilitates the process. The daughter-stars thus formed gradually pass through the reverse process, and each, after becoming a wreath, is transformed into a fine reticulum enclosing the achromatin, as in the parent nucleus (fig. 13, G, h).

When the daughter-nuclei are in the stellar stage, the protoplasm of the cell itself becomes constricted, and the cell is usually quite divided by the time the wreathstage is attained.

This mode of cell-division is known as the “ indirect method ,- and the whole process is termed “ Jcaryokinesis. -

The following schema of the phases of indirect cell-division is modified from Flemming : -

Resting Stage.

/ Wreath form.

|> j Star form.

oq ( Transition phase.

Mother-nucleus. . 1 . Spira.


5. Dispira.

2. Aster. 1 4. Diaster.

-* 3. Metakinesis.

Usually, in segmenting oosperms and in many vegetable cells, the chromatin is less abundant, and the achromatin appears to take a larger share in nuclear division than in tissue-cells. At the stage when the chromatin is equatorially situated (the “equatorial plate,- which is the equivalent of the wreath and aster stage), the achromatin forms a well-marked spindle-shaped bundle of fibres, the apices of which correspond with the centres of the future nuclei. Later, the chromatin separates into two portions, each of which travels along the achromatin fibres to each apex of the spindle, the diaster stage. The intervening achromatin threads break across the middle and are withdrawn.

Van Beneden, who has most carefully studied these phenomena in the oosperm of Ascaris, states that the achromatin spindle is probably always present in the ordinary tissue-cells though difficult of detection ; but it is readily visible in egg-cells when they are properly treated with reagents.

During karyokinesis, the granules of the protoplasm of the cell often become radially arranged with regard to the foci of the daughter-nuclei. These alone can be seen in the living egg (fig. 13, d, e, /), and they should not be mistaken for the chromatin fibres, which are only visible after suitable treatment.

The foregoing is a brief summary of the views generally held respecting karyokinesis. Carnoy, however, gives a somewhat different account. As previously mentioned, he finds all cells to be composed of a fine protoplasmic reticulum enclosing a fluid enchylema , which contains various substances in solution and particles in suspension. The nucleus is similarly constituted, but it possesses in addition a contorted nuclear filament of chromatin. The nuclear reticulum evidently corresponds to the above-mentioned achromatin. According to Carnoy, the convolutions of the nuclear filament very rarely fuse at their intersections so as to constitute an actual network. The wall of nucleus is never formed by the chromatin, but solely at the expense of its reticulum. The latter also forms the nuclear spindle in dividing cells. The polar stars are formed by the reticulum of the cell.

It is probable that there is considerable variation in the method of indirect nuclear division amongst the Metazoa. Yery rarely, the nucleus simply divides in half without forming karyolitic figures. This is known as “ direct nuclear division ,- and has been observed by Blochmann in the embryonic integument of the European Scorpion, ^and by Ranvier in the division of leucocytes in Axolotl.

Our present knowledge appears to warrant the following generalisation concerning the evolution of the nucleus. In some of the simplest of all organisms (Protista) no nucleus has yet been observed ; probably, however, it will be demonstrated that nucleoplasm ( i.e ., chromatin) is present in diffused granules, as Gruber has shown is the case in a few Ciliate Infusoria. A concentration of the chromatin occurs in other Protozoa, forming either several small nuclei or a single large one. In most Protozoa the nucleus divides directly, as it may do, though very rarely, amongst the Metazoa. Physiological differentiation has also acted upon the nucleus of Protozoa, and has resulted in great variation in structure and behaviour. In some Heliozoa and Ciliata the nuclear division bears considerable resemblance to the indirect method characteristic of the tissue -cells of Metazoa.

In segmenting oosperms, the process of nuclear division is not so complicated as it is in the tissue-cells of adults - partly owing, perhaps, to a paucity of chromatin ;

Fig. 14. - Early Stages of Segmentation of Elysia viridis (drawn from the living'egg).

A. oosperm in state of rest after the extrusion of the polar cells ; B. the nucleolus alone has divided; C. the nucleus is dividing; D. the nucleus, as such, has disappeared, first segmentation furrow appears ; E. later stage ; F. oosperm divided into two distinct segmentation spheres - the clear nuclear space in the centre of the aster of granules is growing larger ; G. resting-stage of two appressed spheres ;

H. I. similar stages in/the production of four spheres; K. formation of eightcelled stage.

but it is probable that there is an approach in some cases to the direct method of nuclear division which is so common amongst the Protozoa ; as, for example, in the segmenting oosperm of Elysia (fig. 14, b, c), which may be compared with the nuclear division of the Amoeba (fig. 1, a-c). This irresistibly suggests a retention by certain segmenting oosperms of the ancestral method of nuclear division.

The segmenting typical oosperm was left at a stage in which two segmentation spheres had been formed.

A second series of changes soon takes place, the long axis of the nuclear spindle lying in the same plane as the first, but at right angles to it ; four segmentation- spheres are thus formed, all lying in the same plane (fig. 14, H, 1).

After another short resting-stage, each of the four spheres divides in a manner essentially identical with the preceding. As the nuclear spindle assumes a position at right angles to the two previous directions, the third groove is in a horizontal plane, and a mass of eight cells is produced, four above and four below (fig. 14, k). The segmentation has thus taken place in the three dimensions of space.

In the most regular cases of segmentation, the eight spheres are vertically divided to form sixteen spheres, eight above and eight below (fig. 15). In the next stage, a furrow is formed on each side of the first horizontal or equatorial fissure, and these deepen to produce a mass of thirty-two spheres, consisting of four rows of eight cells each. A sixty-four-celled stage is next reached (fig. 15); but usually, after this, the regular rhythm is lost, and the

Fig. 15. - Segmentation of Oosperm of Frog. [After Eclcer .]

The numbers above the figures refer to the number of segments at that stage.

The dotted lines represent the position of the next furrows or planes of segmentation. The segmentation, though regular, is somewhat unequal owing to the presence of yolk.

order of the segmentation becomes obscure. We thus get the number of cells in each successive stage as follows : - A. 1 ; B. 2 ; c. 4 ; D. 8 ; E. 16; F. 32 ; G. 64 ; n. 00, that is, in geometrical progression. It must, however, be definitely understood that this is not the invariable rhythm of segmentation, but only a generalised type (for example, the Nudibranchs and other Mollusca do not conform to it).

The result of segmentation is the formation of a multicellular body, usually enclosing a central cavity - “ Segmentation cavity - or “ Blastocoel - (fig. 16, a). The body itself is variously termed “ Bias tula - or “ Blastosphere.- Excepting in special cases, the wall of the Blastula consists of a single layer of cells.

Typical Gastrulation

An oosperm devoid of food-yolk (known as alecithal ), or one in which the segmentation is quite regular, has been assumed, but this rarely obtains ; more or less food-yolk is usually present, and its presence is a disturbing factor of great importance. Before, however, discussing the effects of food-yolk upon an oosperm, it will be advisable to continue the history of the simpler condition ; the ova of Echinodermata being particularly suitable for this purpose.

On the completion of the Blastula stage, a slight depression occurs at the pole opposite to that where the polar cells are situated. This is often preceded by a flattening of that pole of the blastula (fig. 1 6, b), the cells of the flattened region assuming a more columnar form, the first indication of a histological differentiation. The invagination deepens until a cup-like cavity is formed (fig. 1 6, d), and eventually there is usually a complete inversion of this pole of the blastula. The growth of the embryo is so rapid that the size and general form of the body is at first little altered by this process, but soon the absolute size is increased and the embryo becomes oval in shape.

A. blastula ; B. later stage, showing the thickening and flattening of the lower pole and appearance of, mesoderm ; C. commencement of gastrulation ; D. later stage ; E. early larval stage with commencing oval invagination ; D. and E. from living embryos, after Metschnikoff.

arc. archenteron ; bl. blastocoel ; bp. blastopore ; ep. epiblast ; hy. hypoblast ; m. mesoblast (mesamceboids) ; m.s. mesoblast cell secreting a spicule ; st. stomodseum.

These phenomena result in the formation of a two-layered embryo, which has an orifice at one end, the Blastopore or primitive mouth, opening into a central sac, the cavity of invagination, Archenteron , or primitive stomach. The outer layer is the Epiblast (Ectoderm), the inner layer lining the archenteron is the Hypoblast (Endoderm), and between these layers is a larger or smaller cavity, which is the more or less reduced segmentation cavity. Such an embryo is known as a Gastrula (fig. 1 6, c).

A modification of ordinary invagination is sometimes met with which is worthy of special notice : - In many Nudibranch Mollusca, the blastula is somewhat quadrate in contour and flattened, being notunlike a book in shape (fig. 17). The gastrula is formed by a kind of rolling over, combined with a slight amount of invagination. An elongated blastopore is the result ; this closes over from behind forwards, the anterior extremity (as indicated by the polar cells) appearing to persist as the mouth.

An extreme example of the method of gastrula-formation by the rolling round of a two-layered flat embryo (the Plakula of Biitschli) is found in the Nematode Worm, Cucullanus. Intermediate stages are, however, to be found in other forms. It will be noticed that in the plakula stage one surface of the embryo is epiblastic, while the other is hypoblastic, and Biitschli compares such an embryo with the problematical organism Trichoplax adhserens [i 7 . E. Schulze ]

Fig. 17. - Gastrulation of Fiona nobilis.

A. oblong flattened blastula (plakula), two embryos in one egg-shell, the lower one seen endwise ; B. gastrula in process of formation ; C. gastrula stage - the slit-like blastopore (bp.) will be still further reduced from behind forwards.

The effect of food-yolk upon these changes has now to be considered. Though, as previously mentioned, food-yolk is of only secondary significance, yet its presence often greatly influences the manner of segmentation and the early development.

Effect of Food- Yolk, Telolecithal Segmentation. - In the formation of a gastrula by simple invagination, the pole of the oosperm opposite the polar cells ultimately becomes the gastric region of the embryo. As might he expected, the yolk, which is merely stored-up nutritive material, is usually almost entirely confined to those cells which have a nutritive function, i.e., the hypoblast cells. Ova in which the yolk is especially concentrated at one pole are termed “ telolecithal As a matter of fact, it is generally possible to distinguish between the two layers in the blastula before invagination commences - the epiblast cells being smaller and more transparent, while those of the hypoblast are larger, rounder, and more opaque. This distinction is often to be observed at still earlier stages ; at the stage of eight segmentation-spheres the four upper cells may be purely epiblastic, while the four lower may be primitive or yolk-hypoblast. According to some investigators, even the first furrow may indicate the first epiblastic sphere ; but the recent researches of Agassiz and Whitman show this to be very doubtful (see p. 268).

It is not difficult to conceive that the distension of the hypoblast-cells with inert matter would cause them to segment with difficulty, and this would hinder their invagination, while an increase in the amount of yolk would still further retard the process, so that a condition might be reached in which it would be impossible for the distended hypoblast cells to be invaginated at all, and the inertness of the large quantity of yolk would allow of only a very few hypoblast-cells being formed. Though the epiblast has been scarcely affected by the increment of yolk in the lower cells, its behaviour with regard to them is necessarily modified, and since the hypoblast cannot be invaginated, the epiblast is obliged to grow round it (fig. 18, a-d).

One effect, then, of the addition of food-yolk to the ovum is to cause the normal method of gas trula- formation by invagination (“ embold-) to be modified into that of overgrowth (“ epiboU -). The segmentation cavity is almost obliterated and the blastopore is greatly reduced, and occasionally may be entirely absent as a distinct orifice (Cephalopoda).

In certain Prosobranch Gastropods, with a large quantity of yolk ( e.g ., Ianthina, Fusus), the oosperm divides into two, and again into four, large segmentation-spheres (fig. 18, A and a ) ; four small cells are next segmented off from the upper poles of these spheres. There are then, at this stage, four small clear epiblast cells and four large opaque yolk- spheres. The yolk again gives rise to four small cells (fig. 18, B and 5), and the first four epiblast cells and the four cells just formed themselves divide, so as to constitute a group of sixteen cells resting upon four large yolkspheres (fig. 1 8, c). By further cell-division a cap of small epiblast cells is formed, which gradually extends round the yolkspheres (fig. 1 8, c, D, and e ), leaving a small uncovered area at the ventral pole, which corresponds to the blastopore of other forms (fig. 1 8, d and f-bjp). The ventral wall of the archenteron (mesenteron of embryo) appears to be formed, or at least partially, by an

Fig. i8. - Segmentation of Two Prosobranchs; to illustrate the effect of the increase of food-yolk.

A-D. Ianthina ; the epiblast cells form a cap which gradually grows round the yolk-cells (primitive hypoblast), a-g. Fusus [ after Boibretzky ] ; a-c. surface views from above ; d. ventral view ; e-g. sections ; bp. blastopore ; int. commencing intestine ; ms. mesoblast ; oes. stomodseum ; p.k. primitive or larval kidney ; sh. shell gland; y, y.liy. yolk-cells or yolk hypoblast.

ingrowth of cells at the posterior lip of the blastopore ; the dorsal wall is certainly produced by the formation of cells (hypoblast) by the yolk-spheres (or primitive hypoblast). The hypoblast cells, especially those situated in the gastric region, actively assimilate the yolk. The blastopore, in some species at least, persists as the mouth, the oesophagus being produced by a further ingrowth of epiblast at that orifice. The segmentation in Nassa, as described by Bobretzky, is somewhat different from the above.

The increase in the amount of food-yolk amongst Invertebrates culminates in the Cephalopoda, in which group segmentation results in the formation of a cap of small cells resting upon the large yolk. This yolk may be regarded as one immense lower-layer segmentation-sphere distended with food-yolk, or better, perhaps, as several fused together (primitive hypoblast). The cap of cells is really the epiblast ; such a cap or layer of cells (one or more cells deep) resting on the yolk is termed a “ blastoderm - Soon nuclei make their appearance on the circumference of the yolk at its upper pole, which nuclei, by aggregating protoplasm round themselves, form the future hypoblast cells. This apparently anomalous proceeding is merely a masked form of segmentation, the protoplasm of the lower cells is so enfeebled by the mass of yolk that it cannot divide; but as segmentation must take place, the nuclei, either alone or with a minute portion of protoplasm, travel to the periphery, and there, by the assimilation of the yolk, build up their cells. A similar phenomenon is common amongst Vertebrates.

In two great divisions of the Vermes, the Platyhelminthes and the Chsetopods, do we find, as in the Mollusca, that the segmentation may be more or less uniform, resulting in a hollow blastula, which further develops into a typical invaginate gastrula ; or, on the other hand, sufficient yolk may be present to cause unequal segmentation, the partial or total obliteration of the blastocoel and the production of an epibolic gastrula.

Amongst the Piatyhelminths, Lineus (fig. 49) and Leptoplana (fig. 50); and the Earthworm and Ehynchelmis (fig. 53) for the Oligochseta illustrate these two types of gastrula formation.

Syncytial Segmentation. - Sedgwick has very recently shown that, even later than the gastrula-stage in the development of the species of Peripatus from the Cape of Good Hope, no definite cell-walls are present. The embryo is, in fact, a syncytium (fig. 19). What corresponds to segmentation in other forms is here effected by the multiplication of nuclei, which aggregate round themselves small portions of the continuous vacuolated protoplasm.

The gastrula arises by a process of epibole, and is at first solid. The archenteron (mesenteron) is simply a large vacuole within a multinucleated mass of protoplasm.

It remains to be seen whether the segmentation-spheres in other developing ova are in all cases separate cells, or whether there may not be a direct or indirect protoplasmic continuity between all the cells of an embryo.

There is a considerable resemblance between such an embryo as that given in fig. 19, B, and the parenchymula of Obelia, fig. 46, E.

Centrolecithal Segmentation. - The food-yolk is not always concentrated within the future hypoblast cells, since amongst the Arthropoda it is usually equally divided between all the segmentation-spheres, the protoplasm of which is mainly peripherally situated. The passiveness of the yolk, generally, not only prevents any entire segmentation, but causes a separation to take place between the pro

A. Blastula with about sixteen epiblast cells. B. Early gastrula stage - the central large vacuole is the commencing archenteron. C. Completed gastrula.

The whole of the protoplasm is directly continuous and largely vacuolated ; the hypoblast is at first non-nucleated.

toplasm and the yolk ; thus an external continuous single layer of cells is formed, within which lies a central mass of yolk, more or less free from protoplasm. Such ova are termed “ centrolecithal.-

The details of segmentation vary somewhat in the Crustacea. In such a comparatively simple case as Callianassa (fig. 20), the nucleus divides in the ordinary manner, without affecting the yolk, till sixteen nuclei are produced ; by this time they have travelled to the periphery of the oosperm, and an external protoplasmic layer is formed, which, on the further multiplication of the nuclei, becomes divided into distinct cells, thus constituting a shell of cells entirely surrounding a solid core of unsegmented yolk.

/ 2 â– 3 4

6 e 7 8

Fig. 20. - Segmentation of Oosperm of Callianassa subterranea. [After Mereschkovjsici.]

1-4. The nucleus divides into 2, 4, 8, 16, and the nuclei travel fi'om the centre towards the surface without affecting the oosperm itself. 5-6. 16-cell stage. 5.

The oosperm possesses a broad external protoplasmic layer, which passes into the central yolk, the former is raised into slight prominences, which correspond to the underlying nuclei ; in 6 the different cells are segmented off from one another, but not from the central yolk. 7. Further cell-division has occurred, and the cells are cut off from the yolk. 8. A single-layered blastoderm of columnar cells surrounds the yolk.

In the Fresh- water Crayfish, however, the greater portion of the yolk itself segments, forming the so-called “ yolk-pyramids - (figs.

Fig. 21. - Blastula and Gastrula of Fresh-water Crayfish (Astacus fluviatilis).

[After Reichenbach and Huxley .]

A. Ovum with the blastoderm, bl., not yet separated from the imperfectly segmented yolk, v. B. Ovum in which the epiblast, ep.b., is completely separated from the yolk, and the archenteric invagination to form the mid-gut or mesenteron, m.g., has taken place, b.p., blastopore.

21 and 22, a). Subsequently the nucleated peripheral portion of each segment breaks away from the yolk-pyramids (fig. 21, b) ; and although the latter retain their segmentation for a long time, they are not to be regarded as having the value of cells, but are merely masses of non-nucleated yolk, with little, if any, active protoplasm.

Amongst the Crustacea, invagination takes place at one pole of the blastula and a gastrula is formed, the residual yolk being of necessity contained w T itkin the segmentation-cavity. The yolk is gradually absorbed by the hypoblast cells, which emit pseudopodial processes for that purpose (fig. 22, e, p). Thus the primitively

Fig. 22. - Figures Illustrating the Development of Astacus.

[ From T. J. Parker after Reichenbach.]

A. Section through part of oosperm during segmentation. B and C. Longitudinal sections during the gastrula stage. D. Highly magnified view of the anterior lip of blastopore to show the origin of the primary mesoblast from the wall of the archenteron. E. Two hypoblast-cells to show the intra-cellular digestion of yolk spheres. F. Hypoblast-cells giving rise endogenously to the secondary mesoblast.

a. Archenteron ; b. blastopore ; c. central yolk mass ; ec. epiblast ; en. hypoblast; n. nuclei; p. pseudopodial process; primary mesoblast; s.rns. secondary mesoblast ; w.y. white yolk ; y. yolk granules ; y.p. yolk pyramids.

small hypoblast cells become greatly distended with yolk. This is what occurs in the Crayfish, and with variations is characteristic of the Crustacea.

The peculiar segmentation of most Insects may be regarded as an extreme modification of the Crustacean type.

B. Chordata - Alecithal Segmentation. - The egg of Amphioxus having very little food-yolk, undergoes entire and regular segmentation up to the stage of thirty-two spheres. The segmentation then becomes slightly irregular, but results in an almost spherical blastula, the cells of the lower pole of which are slightly larger than the remainder. This pole flattens (fig. 23, a) and invaginates to form a wide-mouthed gastrula, in which the segmentation cavity is obliterated (fig. 23, c). The blastopore narrows to a small orifice, and the epiblast becomes ciliated. The gastrula elongates and its dorsal side becomes flattened. Thus the blastopore comes to have a dorso-terminal position (fig. 57, d). A pair of “ hinder-pole mesoderm cells - early make their appearance on the future ventral side of the lip of the blastopore ; their further history is noticed later (p. 61).

Effect of Increase of Food-Yolk. - In the Chordata, as in most Invertebrates, the yolk is stored up in the lower portion of the oosperm, and it is consequently contained within the segmentationspheres of that pole - in other words, within the hypoblast. These

Fig. 23. - Blastula and Gastrula of Amphioxus. [ From Claus after Hatschelc.]

A. Blastula with flattened lower pole of larger cells. B. Commencing invagination. C. Gastrulation completed ; the blastopore is still widely open, and one of the two hinder-pole mesoderm cells is seen at its ventral lip. The cilia of the epiblast cells are not represented.

cells usually have a somewhat complicated history, especially when greatly charged with yolk ; as the primitive hypoblast in that case is only partially concerned in the formation of the digestive tract of the future embryo, it is sometimes termed yolk-hypoblast or lower -layer cells, to distinguish it from the hypoblast of the adult.

The effect of the increase of yolk in the vertebrate oosperm on segmentation and gastrulation resembles in the main that which occurs in some Molluscs. The segmentation is unequal, and the blastocoel is reduced in extent. The epiblast grows round the enlarged hypoblast, and consequently the gastrulation is asymmetrical. The invagination of the hypoblast is but partial, and tends to be increasingly reduced. The primitive blastopore of the true gastrula stage is more and more filled up by yolk- cells (the yolk plug, fig. 24), and it becomes almost if not entirely obliterated as an actual orifice.

The epiblast usually at first consists of a single layer of cells, and its history is simple.

During gastrulation a definite ingrowth of hypoblast occurs at the dorsal lip of the blastopore. This is most marked in forms where there is but a small amount of yolk, and least so in ova with a great deal of yolk. This hypoblast is sometimes spoken of as invaginated hypoblast - or, better, axial hypoblast, as it extends along the median line of the roof of the archenteron. As this tissue gives rise to the notochord, it is called by Hertwig and others Chorda- ent oblast (see figs. 59-64, ax. hy .)

The sides and floor of the archenteron are bounded by the yolkcells in forms which have a relatively small amount of yolk. In these ova the yolk cells which immediately bound the archenteron are usually directly transformed into the definite hypoblast of the digestive portion of the alimentary canal or mesenteron. These cells may be called the digestive or gut-hypoblast, or simply hypoblast; this is the Darm-entoblast of the Germans (figs. 60-65, hy). These cells are distinctly different in character from the axial hypoblast. In forms with a great deal of food yolk these cells have a slightly different origin, as will be shortly described.

The remaining yolk-cells may be termed the yolk-hypoblast; and, like the unsegmented yolk, they simply serve as pabulum for the developing embryo.

In telolecithal ova with a large amount of yolk, only a small cap of primitive hypoblast-cells is formed ; in this case these are usually termed lower-layer cells. These lower-layer cells more or less entirely surround the segmentation cavity, and themselves rest upon the large unsegmented yolk (figs. 25, 26, 31).

The segmentation cavity or blastocoel in all alecithal ova is bounded on the one hand by the epiblast and on the other by the hypoblast (figs. 16, 19, 23, 24). Even in such an extreme telolecithal type as the Bird, Duval has shown that the same condition obtains in a very early stage (fig. 29). Thus the encroachment of the lower-layer cells round the segmentation cavity in the Elasmobranchs (fig. 26) is a purely secondary condition of no special import.

As will be described in its appropriate place, the primitive hypoblast also gives rise to the main mass of the mesoblast. It is convenient to restrict the name of archenteron to the cavity of the early gastrula stage, and after the formation of the mesoblast to term the corresponding cavity the mesenteron (that is, the hypoblastic portion of the alimentary canal comprising the pharynx, oesophagus, stomach, and intestine). The reasons for this will presently appear sufficiently obvious.

The effects of the gradual increase of food-yolk in oosperms will now be illustrated in more detail.

In the Lamprey (figs. 60, 6i), and slightly more so in the Newt (figs. 58, 59), enough yolk is present to cause the cells of the primitive hypoblast to be larger than those of the epiblast, and to induce an asymmetrical invagination. The axial hypoblast

Fig. 24. - Blastula and Gastrula Stages of the Frog (Rana temporaria). [After Gotte .]

A. early blastula stage ; B. late blastula ; C. commencing gastrula ; D. later stage; E. completed gastrula stage, longitudinal section to one side of the median line.

al. archenteron (mesenteron) ; hi. blastoccel ; blp. blastopore ; ep. deeper, and ep'. epidermal layer of epiblast ; Tty. hypoblast ; to. dorsal, and to', ventral mesoblast ; n.p. neural plate of future brain ; y.Tty. yolk hypoblast.

is very distinct, and the yolk-cells forming the sides and floor of the archenteron are transformed into the hypoblast of the mesenteron.

More yolk is present in the Frog -s oosperm, but the first stages of segmentation are only slightly affected by this increase. The third furrow, instead of being equatorial, is nearer to the upper or black pole of the egg (figs. 1 5 and 24) : as this pole is less burdened with yolk than the lower pole, it is only to be expected that segmentation should be more rapid and complete there. In the final blastula stage, the segmentation-cavity is bounded above by two layers of epiblast, an epidermal and an inner nervous layer,, the latter eventually becoming three cells thick.

The epiblast gradually extends over the surface of the primitive hypoblast and the uncovered portion (yolk-plug, anus of Rusconi) is reduced to a small round white spot, entirely surrounded by the darkly pigmented epiblast (fig. 24, hip). The posterior extremity of the future embryo is formed by the dorsal lip of the blastopore. At this point an ingrowth of cells occurs (fig. 24, D. hy), which constitutes the hypoblastic dorsal wall of the mesenteron. The ingrowth of the hypoblast continues, and a slit-like archenteron appears between it and the yolk hypoblast. Meanwhile the segmentation-cavity has been pushed to one side, and eventually disappears. The gastrula in the Frog is thus formed partly by invagination ( emhoU ), partly by overgrowth ( epiboU ).

In some forms (e.g., Sturgeon) the primitive hypoblast extends up the sides of the segmentation-cavity and helps to form its roof.

Given more yolk, further complications would arise. Balfour has drawn an ideal type (fig. 25), intended to illustrate the passage from the Amphibian to the Elasmobranch gastrula. The segmentationcavity is entirely surrounded by lower-layer cells, and below these again is the unsegmented yolk penetrated by a protoplasmic reticulum. This is merely an exaggeration of the tendency to a separation which occurs in the primitive hypoblast between cells containing less from those containing more yolk. On reference to the Frog -s ovum in fig. 24, c, a mass of smaller primitive hypoblast cells (m) will be seen at the lips of the blastopore, which corresponds to the cap of lower-layer cells of fig. 25, A.

Asymmetrical invagination is assumed to occur in this ideal type, the invaginated hypoblast forming the roof of the archenteron, while a portion at least of its floor is derived from cells which form round those scattered nuclei (fig. 25, b, n) which appear below the archenteron, and which are themselves derived from the nuclei of the primitive yolk-cells.

Telolecithal Segmentation and Gastrulation. - Owing to the immense amount of yolk in the oosperm of an Elasmobranch, segmentation is only very partial. The protoplasm of the oosperm mainly segregates to the upper pole, and here also the yolk granulesare of smaller size : this area is termed the germinal disc. A delicate protoplasmic network extends throughout the whole of the yolk.

Segmentation commences by a groove extending nearly across the germinal disc ; this is crossed by a second at right angles to it ; subsequently other grooves appear, and horizontal fissures convert these into polygonal masses, each of which is provided with a nucleus, and is, in fact, a segmentation sphere (compare fig. 27). Eventually a circular cap (blastoderm) of minute cells is formed,

Fig. 25. - Three Diagrammatic Longitudinal Sections through an Ideal Type of Vertebrate Embryo, Intermediate in the Mode of Formation of its Layers between Amphibia or Lamprey and Elasmobranchii.

[ From Balfour .]

al. mid-gut ; ep. epiblast ; hf. head-fold ; Tiy. hypoblast ; m. mesoblast ; n. nuclei of the yolk ; nc. neural canal ; sg. segmentation-cavity.

of which an upper layer is distinctly columnar and constitutes the epiblast, while the underlying mass of rounded or polygonal cells is the primitive hypoblast or lower-layer cells. A cavity, the segmentation-cavity, soon occurs within the latter. Although the blastoderm is sharply defined from the underlying yolk, the latter must be regarded as essentially homologous with the lower-layer cells, the main difference being that the primitive hypoblast segments into definite cells in that area where there is sufficient protoplasm, whereas in the greater portion of its mass it is unable to segment, owing to the preponderance of food-yolk. Nevertheless, the nuclei belonging to the latter divide, and the nuclei thus produced (figs. 25, B, c, 26, n) may be seen at the upper surface of the so-called yolk. In process of time these free nuclei form cells, of which some pass into the blastoderm, and others will constitute the floor of the mesenteron.

At one region the blastoderm projects slightly from the yolk, forming what is termed the embryonic rim. At this spot the epiblast, bending round the rim, imperceptibly passes into a columnar layer (hypoblast proper), which is being differentiated from the lower-layer cells (fig. 26). This differentiation extends anteriorly,

Fig. 26. - Longitudinal Section through the Blastoderm ok an Elasmobranch during Gastrulation. [Modified, from Balfour.]

a. archenteron (mesenteron) ; e.r. embryonic rim ; ep. epiblast ; liy. hypoblast

- the line points to the spot where the invagination occurs at the dorsal rim of the blastopore ; 1. 1. lower-layer cells or primitive hypoblast ; m. mesoblast ; k. nuclei of the yolk ; s.c. segmentation cavity.

In the corner a nucleus of the yolk is shown very highly magnified, and a portion of the protoplasmic network connected with the nucleus.

and a space is left between the developing hypoblast and the underlying yolk. The embryonic rim is the dorsal lip of the blastopore ; the anteriorly progressive differentiation of the lowerlayer cells into true hypoblast corresponds with the gastrula invagination of other types, and the cavity between the hypoblast and the yolk is the archenteron or the future mesenteron. Those lower-layer cells which do not participate in the hypoblast constitute the mesoblast (see p. 67).

The blastopore proper is situated at the posterior end of the embryo. The blastoderm gradually extends over the yoke in every direction except immediately behind the embryo, which thus comes to be situated at the end of a bay or sinus. In process of time the yolk is entirely surrounded by the blastoderm, the edges of which unite in a linear manner (primitive streak) behind the embryo (fig. 35, B, bl).

The segmentation of the Fowl -s egg corresponds sufficiently closely with that of an Elasmobranch to obviate a description. Fig. 27 illustrates a superficial view of the segmenting blastoderm, and figs. 28-31 show sections at various stages of segmentation. Duval states that the segmentation-cavity appears very early (fig. 29) ; it is bounded above by a single layer of epiblast-cells, and at first below by a single layer of primitive hypoblast cells ;

Fig. 27. - Surface Views of Six Stages in the Segmentation of a Fowl -s Oosperm. [ From K'olliker after Coste. ]

All the eggs were taken from the lower portion of the oviduct. The shading outside the germinal disc represents the yolk. Diameter of the germinal disc, 3 mm.

1. Earliest stage ; a. the first furrow. 2. Stage of four imperfect cells separated by furrows. 3. Stage of nine meridian furrows and cross-furrows have also appeared, which divide the disc into nine large peripheral cells and seven small central cells. 4. A later stage nuclei are to be seen in the central clearer cells ; the cells are polygonal through mutual pressure. 5. Further stage in segmentation ; the cells gradually decrease in size towards the centre. 6. Completion of segmentation ; the blastoderm consists of an upper layer of small cubical cells (epiblast) and a lower-layer mass of larger cells.

but the latter soon becomes composed of several layers and the segmentation- cavity is obliterated. The blastoderm of a newlylaid egg (figs. 30 and 31) consists of a definite epiblastic layer and an inferior irregular mass of rounded cells, the primitive hypoblast (lower-layer cells), which lies loosely on the yolk. In the upper surface of the yolk free nuclei occur, which have the same significance as those of the Elasmobranch ovum, i.e., they represent primitive hypoblast cells whose walls are not limited. After incubating for an hour or two the latter mass is differentiated into a lower stratum of flattened cells, the hypoblast proper, and scattered mesoblast cells lying between the epiblast and hypoblast. The

Fig. 28. - Semi - Diagrammatic Section through a Fowl -s Blastoderm corresponding to No. 3, Fig. 27. [Modified from Kolliker and Balfour.]

B. white yolk spheres ; C. isolated yellow yolk-sphere.

bl. blastoderm ; w.y. white yolk, the upper finely granular layer of which is the seat of cellformation ; y.y. yellow yolk.

Fig. A. is cut off just where the white yolk is expanding to form the central mass.

imperfect cavity (sub-germinal cavity) between the hypoblast and yolk corresponds with the archenteron of other forms.

Duval has figured a longitudinal section of the blastoderm of a

Fig. 29. - Section through the Blastoderm of an Unfertilised newlylaid Fowl -s Ovum in the Blastula Stage. [After DuvaL]

ep. epiblast ; n. free nuclei in the yolk, which aggregate protoplasm round themselves to form primitive hypoblast cells; s.c. segmentation-cavity (blastocoel). The white yolk is left blank ; it rests upon the coarser yellow yolk, represented by dots.

Canary about this stage (fig. 32). The slit between the blastoderm and the yolk is at the posterior end of the future embryo, and corresponds with the slit-like archenteric invagination of the Lamprey

Fig. 30. - Semi-Diagrammatic Section through the Germinal Disc of a Fowl during the Later Stages of Segmentation.

The central cells are the smallest owing to rapid segmentation ; the large peripheral cells of the epiblast rest directly upon the white yolk ; ep. epiblast ; 1 . 1 . primitive hypoblast ; n. free nuclei in the yolk ; w.y. white yolk.

(fig. 6o) or Frog (fig. 24), or better still, with stage B, or one somewhat earlier, of the diagram of the ideal vertebrate (fig. 25). The yolk of the Bird is clearly homologous with the yolk-cells of the Frog ; and the surface of the yolk uncovered by the blastoderm of the one corresponds with that area of the yolk-cells not surrounded by the epiblast in the other.

It must be remembered that the blastoderm at this stage covers only a very small extent of the surface of the ovum, and that figures so greatly enlarged as figs. 30-32 rather tend to give an exaggerated idea of the relative size of the blastoderm with regard to the

Fig. 31. - Portion of a Section through an Unincubated Fowl -s Oosperm. [From Klein.}

The thin upper layer a. (epiblast) is composed of a single layer of columnar cells ; at the edge it rests directly upon the white yolk ; b. irregularly disposed lower-layer cells (primitive hypoblast) ; c. the larger so-called formative cells resting on the white yolk ; /. archenteron ; the segmentation-cavity is the nearly obliterated space between the epiblast and hypoblast.

rest of the oosperm. The blastoderm of this stage is considerably smaller than the central pale area in fig. 6.

In a surface view of the blastoderm of a newly laid Fowl -s egg, a central transparent nearly circular space (area pellucida) is seen surrounded by an opaque ring (area opaca). The former appearance is due to the fact that the blastoderm is separated from the yolk by a shallow space filled with a fiuid, whereas the area opaca

Fig. 32. - Section through the Blastoderm of an Unfertilised Canary -s Ovum tn the Gastrula Stage. [After Duval.}

Bp. blastopore ; ep. epiblast, below which is the primitive hypoblast or lower-layer cells ; n. free nuclei, which will form primitive hypoblast cells.

rests on the yolk itself. The embryo is developed within the area pellucida alone (see fig. 6). The area opaca gradually extends over the whole surface of the ovum enclosing the yolk, its lowerlayer of primitive hypoblast gradually assimilates the enclosed yolk. That portion of the area opaca immediately surrounding the area pellucida develops a large number of blood-vessels and is known as the area vasculosa. Nutritive matter is transmitted to the blood by the hypoblast of the area opaca, and by it conveyed to all the regions of the body of the embryo.

To anticipate, as the embryo is being formed, an anterior, and later a posterior, fold in the blastoderm make their appeareance, which mark the anterior and posterior extremities of the embryo ; they are known as the head and tail folds. The head-fold travels backwards and the tail-fold forwards in such a manner as to constrict the embryo from off the yolk. Less marked lateral folds also appear. Eventually the embryo is quite constricted off the yolk, so that it is merely connected with the latter (or yolk-sac, as

Fig. 33. - Surface View of the Pellucid Area of the Blastoderm of a Fowl of Twenty Hours. Magnified 24 diameters. [From Kolliker after His.]

Ao. area opaca ; Ap. area pellucida ; Pr. primitive streak ; vAf. head-fold.

it is now termed) by a narrow stalk. The development of the embryonic structures known as the amnion and the allantois will be considered in another section (p. 78).

The Primitive Streak. - The first noticeable sign of incubation in the blastoderm of the Amniota when viewed from above is the appearance of an opaque band which extends some distance forwards from the posterior margin of the area pellucida. This is the primitive streak, and its opacity is due to the presence of a greater thickness of cells than occurs elsewhere. Shortly after the primitive streak is formed a shallow groove (the primitive groove) extends along its whole length. The area pellucida soon becomes oval in outline, and the primitive streak assumes a more central position (fig. 33).

In a transverse section through the primitive streak on its first appearance, the blastoderm is seen to consist of an external layer of columnar epiblast ; inferiorly there is a layer of flattened cells (hypoblast) which extends from one yolk-wall to the other. Between those two layers is the third germinal layer or mesoblast. On each side, especially close to the yolk or germinal wall, the mesoblast cells are loosely heaped up, whereas in the centre they form a dense mass, which, appearing through the epiblast, gives its characteristic appearance to the primitive streak. In the region of the primitive streak a fusion of the epiblast with the axial

Ftg. 34. - Transverse Section through the Anterior End of the Primitive Streak of a Fowl -s Blastoderm about the Age of Fig. 34. [From Balfour . ]

Showing the rounded mesoblast cells arising from the primitive streak and the stellate cells of hypoblastic origin.

ep. epiblast ; liy. hypoblast ; m. mesoblast ; pv. primitive groove ; yh. yolk of germinal wall.

mesoblast always occurs (figs. 34, 43), and a complete fusion of all the layers occurs in a limited area in some forms (fig. 43, c).

At a slightly later stage, on the appearance of the primitive groove, the epiblast and hypoblast have much the same character as before. The axial or primitive-streak mesoblast has, however, a greater lateral extension (fig. 34), and is readily distinguishable from the other mesoblastic cells, which have now assumed a stellate character.

Although, for the sake of convenience, an account of the formation of the mesoblast is relegated to another chapter, it is impossible to avoid referring to this germinal layer in this place, as its history is so closely connected with that of the primitive streak.

The changes which have occurred are briefly these. The lowerlayer cells or primitive hypoblast have become differentiated into an inferior sheet of flattened cells (hypoblast) and an intermediate tissue of scattered cells (mesoblast). In the mesial line behind the future embryo, the epiblast by rapid cell-division (proliferation) has given rise to a linear mass of axial mesoblast, which later widens out into a lateral sheet of cells.

Nature of the Primitive Streak. - Very much has been written concerning the significance of the primitive streak, but it

Fig. 35. - Diagrams Illustrating the Position of the Blastopore, and the Relation of the Embryo to the Yolk in various Meroblastic Vertebrate Oosperms.

A. Type of Frog. B. Elasmobranch type. C. Amniotie Vertebrate. [ From Balfour.']

bl. primitive streak, caused by concrescence of the lips of the blastoderm behind the embryo ; mg. medullary or neural groove in the centre of the neural plate; ne. blastopore; yk. part of the yolk not yet enclosed by the blastoderm.

is now generally admitted that it represents the fusion of the lips of the blastoderm, which meet behind the blastopore.

The embryo develops subsequently in front of the primitive streak, the posterior end of the one coinciding with the anterior end of the other (figs. 100, 101). At the anterior end of the primitive streak a pit usually occurs, which frequently perforates the blastoderm, and corresponds to the blastopore. In the Lizard, Weldon finds that the primitive hypoblast first takes on the character of the permanent hypoblast at the anterior border of this pit (blastopore), in this respect recalling the development of the so-called invaginated hypoblast of an Elasmobranch. In the primitive streak of a Lizard all the three layers are fused together.

Fig. 35 graphically illustrates how Balfour assumes the primitive streak to have originated. Fig. A represents a view of a Frog -s oosperm at a slightly later stage than Fig. 62 ; the yolk-cells are still slightly uncovered. An Elasmobranch -s oosperm is shown at R ; owing to the large increase in the yolk the latter is largely uncovered, but the blastoderm gradually fuses in the middle line behind the

Fig. 36. - Portion of the Blastoderm of an Abnormal Fowl -s Ovum of Eighteen Hours - Incubation. [After Whitman .]

a 0. ai-ea opaca, a small portion of whscli lias alone been shaded ; a.p. area pellucida; p.g. primitive groove; p.p.g. posterior prolongation of primitive groove; p.s. primitive streak ; m.n. marginal notch.

posterior end of the embryo, so that the latter comes to be centrally situated in the blastoderm. By an abbreviation of this process in the Sauropsida, the primitive streak itself is developed towards the centre of the blastoderm (fig. 35, c). This diagram indicates the area pellucida with the developing embryo surrounded by the area opaca, and beyond this again is the uncovered yolk. The edge of the area opaca is often notched immediately opposite to the posterior end of the primitive streak ; and Whitman has described an abnormal form of a Fowl -s blastoderm (fig. 36) in which the primitive streak extended right across the area opaca to the marginal notch, which is plainly a reversion to a stage analogous to that figured in fig. 35, b.

Since the epiblast becomes continuous with the primitive hypoblast at the lips of the blastopore of the Frog, it follows that on the junction of such lips there would be a fusion of the layers :

Fio. 37. - Section through the YolkBlastopore of Oosperm of Nightingale. [After Duval.] ep. epiblast ; hy. hypoblast ; m. mesoblast ; y. yolk, forming a yolk-plug in the blastopore.

this actually occurs in the Lizard. If a differentiation previously took place between the mesoblast and permanent hypoblast, the fusion of the layers would be less evident. It is then not surprising that, in such an abbreviated development of the primitive streak as we find in the Fowl, the hypoblast is already separated as a distinct layer (fig. 34). A comparison of a transverse sec

Fig. 38. - First Stages of Segmentation of a Rabbit -s Oosperm : Semi-Diagrammatic. [From Quain; drawn by Allen Thomson after E. Van Beneden's description.]

a. two-cell stage ; b. four-cell stage ; c. eight-cell stage ; d, e. later stages of segmentation, showing the more rapid division of the outer -layer cells and the enclosure of the inner -layer cells ; ect. outer-layer cells ; ent. inner-layer cells ; pgl. polar cells ; zp. zona pellucida.

tion of an uncoalesced primitive streak of a Nightingale (fig. 37) with the almost completed blastopore of a Frog (fig. 62, c, and d) will further tend to demonstrate the complete homology of the two stages. Duval has found traces of a similar condition in some Fowls - eggs, and the same may also be seen in a transverse section of the blastopore of a Lizard. Mitsukuri and Ishikawa have very recently described a perfectly similar stage in the Turtle (Trionyx).

Segmentation of the Mammalian Oosperm - Blastodermic Vesicle. - So far as is known, the oosperm of all the higher Mammals (Eutheria) undergo total and, at first, regular segmentation. In the Eabbit, according to Van Beneden, the first furrow separates what he terms the epiblast from the hypoblast ; but it will be better, for the present, to call them, with Heape, the outer and inner layer cells (fig. 38). Each sphere divides into two, and these into two more spheres.

Fig. 39. - Sections through the Oosperm of the Rabbit during the Later Stages of Segmentation, showing the Formation of the Blastodermic Vesicle. [ From Quain, after E. Van Beneden .]

a. the outer-layer cells have entirely surrounded the inner-layer cells, except at one spot, the “blastopore- of Van Beneden; b. the enclosure is complete, and fluid is beginning to accumulate to form the vesicle ; c. and d. later stages ; ect. outer layer ; ent. inner layer ; zp. zona pellucida.

In the eight-celled stage (fig. 38, c) one inner-layer cell is more centrally situated. Further segmentation results in a cap of smaller, more transparent outer-layer cells surrounding a solid mass of granular inner-layer cells (fig. 38, e). Eventually the latter are entirely surrounded, except at one spot, the so-called “ blastopore - of Van Beneden (fig. 39, a), but this is also, rapidly closed over.

The outer layer next enlarges so as to form what is termed the blastodermic vesicle, while the inner layer remains attached as an irregular mass to that pole of the ovum where Van Beneden -s “ blastopore - was situated. Later the blastodermic vesicle increases in size, and is bounded by a single layer of flattened outerlayer cells, and the inner layer forms a small disc of cells attached to the upper side of the vesicle (fig. 39, d). In the Bat, however, the “blastopore - of Yan Beneden is larger, and persists until there is a considerable cavity in the blastodermic vesicle (fig. 40).

Fig. 40. - Section through the Blastodermic Vesicle of a Bat. [After Van Beneden and Julin.]

The outer-layer cells should be represented with large granules. The inner mass consists of finely granular protoplasm with imbedded nuclei, but it is impossible to distinguish the limits of the cells.

It would appear that this inner mass not only gives rise to a layer of flattened hypoblast cells by a differentiation of its inferior surface, but that it also gives rise principally, if not entirely, to the epiblast of the embryo. As will be immediately shown, the inner-layer disc corresponds with the early blastoderm of other Vertebrates, the greater portion of the outer-layer cells forming the external wall of the blastodermic vesicle ; but they also extend

Fig. 41. - Diagrammatic Section of a Mammalian Blastodermic Vesicle, in which the Primitive Invagination of the Blastoderm is Rectified, and the Covering Cells have Extended over the Blastoderm.

ep. epiblast of future embryo ; ep'. non-embryonic epiblast, or the epiblast of the area opaca ; liy. primitive hypoblast ; y.s. yolk-sac.

as a covering layer (Deckenschicht) completely over the blastoderm proper. An extension of the hypoblast subsequently forms a second layer underlying the epiblast of the blastodermic vesicle.

The oosperm appears at this stage (fig. 41) as a vesicle, of which the upper half is three-layered, the layers being the covering layer, the epiblast, and the now differentiated hypoblast (fig. 41, hy), while the lower half consists for some time of a single layer of epiblast. The covering cells, however, soon disappear, either entering into the formation of the embryonic epiblast or become attached to the decidua (see p. 92) ; in the latter case they would not form any portion of the embryo proper.

A translucent circular patch next appears at what corresponds with the upper pole of other oosperms (fig. 42), this embryonic area soon becomes ovoid and is homologous with the area pellucida of the Fowl. A primitive streak with its groove makes its appearance at the posterior end of the area. In the Mole, according to Heape, the blastoderm is perforated immediately in front of where the primitive streak is commencing to form (fig. 43, a) ; later this spot is marked by a small down-growth of the epiblast, which really corresponds with the anterior border of the blastopore. Somewhat more posteriorly a complete fusion takes place between the epiblast and incipient mesoblast (fig. 43, b), while at the posterior end of the streak a complete fusion of all the layers occurs (fig. 43, c) ; but the three layers are distinct beyond the streak'itself.

Fig. 42. - Rabbit -s Oosperm Seven Days after Impregnation. 3.47 mm. in length. Side view deprived of its envelopes. Magnified about 10 diameters. [ From KdlliJcer.]

ag. Area pellucida, or embryonic area ; ge. inferior limit of the hypoblast ; below this line the blastoderm consists solely of a single layer of epiblast.

The similarity of a Mammalian blastoderm at this stage with that of a Bird, or especially of a Lizard, is very striking, and it led Balfour to propose the view that the Mammalian ovum originally possessed a large quantity of yolk, since the blastodermic vesicle is clearly homologous with the yolk-sac and contains a coagulable fluid comparable to some extent with the yolk. The primitive streak is the same structure in both Sauropsids and Mammals, that is, it represents a vanished blastopore.

It has since been proved by Haacke and Caldwell that the previously known but discredited fact was true that the Monotremata are oviparous, and that the eggs are in all essential points perfectly comparable with those of Reptiles. Thus Balfour -s deduction from purely embryological data has been verified.

The primitive possession and the subsequent loss of food-yolk must be taken into consideration when dealing with the early stages of the development of the higher Mammalia. It has already been demonstrated how the presence of a large quantity of yolk is a

Fig. 43. - Sections through the Blastoderm of a Mole (Talpa). [After Heape.]

A. Longitudinal section through the middle line of part of an embryonic area in which the primitive streak has commenced to form ; the blastoderm is perforated in front of the primitive streak. B. Transverse section through the middle of a well-developed primitive streak ; the epiblast and mesoblast are fused, but the hypoblast is distinct ; the mesoblast here extends beyond the embryonic area. C. Same as B, but through the hind-knob of the primitive streak. All the layers are fused in the embryonic area, but are distinct beyond.

bp. blastopore; ep. epiblast; hy. hypoblast; m. mesoblast; primitive streak.

disturbing factor; the subsequent loss of this would necessarily still further complicate matters.

Suggested Explanation of Mammalian Segmentation. - Tlie following suggestions, previously published by the author, may perhaps tend to elucidate the apparent anomaly of the process of segmentation in a Mammalian oosperm. A somewhat similar hypothesis was independently arrived at by Minot.

Fig. 44. - Diagrammatic Transverse Section


the Oosperm of a Hypothetical Primitive Mammal.

ep. epiblast of future embryo; ep'. non-embryonic epiblast, which is surrounding the yolk ; hy. primitive hypoblast ; y. yolk.

The oosperm of a hypothetical primitive mammal (the Monotreme -s oosperm is doubtless very similar to this) in which the yolk is still present is represented in fig. 44. The blastoderm, which rests upon the yolk, consists of an epiblastic layer and a mass of lower-layer cells ; the yolk is being surrounded by the non-embryonic epiblast [ep').

An oosperm in which the yolk is supposed to have been lost is shown in fig. 45, a \ and, owing to its absence, the yolk blastoderm or non-embryonic epiblast has precociously completed the blastodermic vesicle, and the blastoderm has sunk into the cavity of the now empty yolk-sac. This figure practically corresponds with the oosperm of the Bat figured above (fig. 40).

The inner mass is thus composed from the first of epiblast and primitive hypoblast, and the break in the outer layer (“blastopore- of Yan Beneden) merely indicates the passage from the yolk blastoderm or area opaca to the embryonie blastoderm or area pellucida.

The increase of yolk during the evolution of a meroblastic from a primitively lioloblastic oosperm results in a growth of the epiblast over the yolk. This also occurs in the Monotremata ; but even after the yolk was lost this long-inherited tendency would persist ; and since the yolk is absent, the completion of the overgrowth would necessarily be very precocious ; so it comes about that in the Rabbit it is completed in about seventy hours (fig. 39).

In fig. 45, b, the epiblast has grown over the embryonic area, forming the covering cells (Deckenzellen). Lastly, the invagination of the embryonic area is rectified

Fig. 45. - Diagrammatic Transverse Sections through a Hypothetical Mammal Oosperm.

A. Stage corresponding to figs. 40, a, and 41. The yolk of the primitive mammalian oosperm is now lost. B. Later stage, corresponding to fig. 39, c and d.

The non-embryonic epiblast has grown over the embryonic area to form the covering cells.

ep. epiblast of embryo ; ep'. epiblast of yolk-sac ; hy. primitive hypoblast ; y.s. yolk-sac or blastodermic vesicle.

(fig. 41), and there is a double -layered oosperm, the covering cells forming the spurious third layer, which misled Yan Beneden into describing the oosperm at this stage as consisting of the three primitive germinal layers.

The completion of gastrulation, which in Vertebrates with meroblastic (telolecithal) oosperms is indicated hy the appearance of the primitive streak, marks the close of the last stage of development which is common to all the Metazoa.

C. Gastrulation by Immigration and Delamination - All the above-mentioned cases of gastrula formation may be reduced to one common type - invagination. There is, however, another series of phenomena which equally result in the formation of a double-layered from a single-layered embryo, which only occurs amongst the Hydromedusae, and possibly in some Sponges.

The development of Obelia (fig. 46), which has been recently studied by Merejkowsky, will serve as a type. The segmentation is regular, and results in a large oval blastula, the cells of which are equal in size and ciliated ; the wall is also stated to be perforated by small pores. The embryo next becomes somewhat narrowed at the posterior end.* One by one the cells at the extreme hinder end of the embryo become amoeboid and pass into the segmentation-cavity and wander about, congregating at first chiefly at the hinder extremity ; eventually the entire segmentationcavity is filled up by a cellular network formed by the fusion of the pseudopodia of these endoderm cells. Metschnikoff proposes the name “ parenchymula- for such an embryo, which is formed of an ectodermal layer and a central solid mass of endodermal cells,

Fig. 46. - Formation of the Planula of Obelia. [After Merejkowsky .]

A. Longitudinal section of a blastula with a few scattered endoderm cells, chiefly at the hind-end. B. Posterior extremity of a slightly earlier stage, showing the proliferation of the terminal cell ; the resulting endoderm cells immigrate into the segmentation-cavii y. C. Surface view of a small area of a blastula with two pores. D. Section through a pore. E. Planula in which the segmentation-cavity is filled up with branched endoderm cells. F. Two-layered ciliated planula, with a definite archentric cavity, but no mouth. After a short free life the planula becomes fixed.

but without a mouth. The term “ planula - is usually applied to this and the succeeding stage. The endoderm now applies itself to the ectoderm as a definite layer, leaving a central cavity ; the archenteron and the free-swimming planula is a ciliated elongated two-layered embryo, also destitute of a mouth. After a short free existence, the planula attaches itself by its anterior end, the ectoderm secretes a perisarc, a mouth and tentacles appear, and the hydroid stage commences.

In this type the endoderm is formed by immigration, which is positively stated to occur only at one pole of the blastula.

W. K. Brooks describes the planula of the Hydromedusoid

The terms u anterior- and. “posterior- have reference merely to the direction of progression of the larva.

Eutima as transparent and pear-shaped ; he actually witnessed the inner ends of some of the ectoderm cells splitting off (delaminating) to form the endoderm ; this takes place most rapidly at the small end, but endoderm cells are formed over the whole inner surface, and they arrange themselves in a single layer one cell thick around a central digestive cavity.

In the specialised Hydromedusa Geryonia (fig. 47), Eol describes the formation of the endoderm by delamination from all the primitive cells of the blastula ; a mouth subsequently opens into the gastric cavity thus formed.

These three types appear to form a series, of which the first can scarcely be doubted to be the most primitive ; and the formation of the endoderm by delamination may be regarded as derived secondarily from immigration.

Fig. 47. - Sections through Three Stages in the Segmentation of Geryonia. [After Fol . ]

A. Stage of thirty-two cells ; each cell is divided into an external, finely granular layer (indicated in the figure by shading) and an inner layer of clearer protoplasm. B. Later stage, in which the outer portion of the cells has given rise to a second cell, and the inner portions exhibit a protoplasmic reticulum. C. The endoderm (hypoblast) has been formed by a delamination of the inner portion of the cells ; it now encloses the alimentary cavity (archenteron). The outer cells constitute the ectoderm.

In some Hydrozoa segmentation is stated to result in a solid mass of cells (Morula), the outer layer (ectoderm) of which is next split off from the internal solid mass. A central cavity appears in the latter ; the cells bounding it are ultimately arranged as a single layer of endoderm.

Although there is still difference of opinion on the subject, the present evidence points to the view that immigration is closely allied to invagination, of which, indeed, it may be regarded a special form. Delamination has probably arisen through precociousness in the formation of the endoderm.

D. Segmentation and Gastrulation of Sponges. - There is so much diversity in the development of Sponges that it is at present impossible to reduce the variations to one common type, as can be done in other groups of animals.

Segmentation, which is fairly regular, results in the formation of a hollow blastula, the further development of which varies accordingly as a planula or an ampliiblastula is formed.

The Planula . - The planula is a solid embryo consisting of an external columnar flagellate ectoderm and a central gelatinous substance containing amoeboid cells. On becoming fixed the ectodermal cells are greatly flattened and lose their flagella, and a central cavity appears lined by a distinct endodermal epithelium, which in their turn become flagellate. The intermediate tissue persists as the mesoderm.

The walls of the central cavity bud off flagellate chambers into the mesoderm, and all the endoderm, excepting that which lines the chambers, is converted into a platelike epithelium.

By perforations in its walls oscula and pores arise, and by various foldings of different parts the adult stage is reached. The structure of Sponges is, as a rule, greatly complicated by accelerated and retarded growth combined with concrescence and imperfect gemmation.

The Amphiblastula . - The amphiblastula is a hollow larva, one hemisphere being formed of granular amoeboid cells, the other of columnar flagellate cells. The latter (endoderm) eventually are invaginated within the former (ectoderm).

The hitherto free-swimming gastrula becomes attached by its blastopore. A middle layer (mesoderm) is now developed, apparently from the ectodermal cells [Metschnikoff], but this requires confirmation. The complications which succeed differ according to the group to which the embryo belongs.

Other methods of embryo-formation have been described, but the two above mentioned may be taken as fairly representative ; the second appears to be almost confined to the Calcispongise.

In all cases the spicules are of mesodermal origin. Nerve-cells and sense-cells have quite recently been described in a few forms by Stewart, Yon Lendenfeld, and Sollas (p. 165), these are stated by Yon Lendenfeld to be of mesodermal origin, as are also the unicellular glands and the muscle cells.

The Porifera form such a distinct and divergent group of the Metazoa that their development appears to have no direct bearing upon that of other Metazoa.

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Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Haddon 1887: Chapter I. Maturation and Fertilisation of Ovum | Chapter II. Segmentation and Gastrulation | Chapter III. Formation of Mesoblast | Chapter IV. General Formation of the Body and Appendages | Chapter V. Organs from Epiblast | Chapter VI Organs from Hypoblast | Chapter VII. Organs from Mesoblast | Chapter VIII. General Considerations | Appendix A | Appendix B

Cite this page: Hill, M.A. (2019, July 19) Embryology Book - An Introduction to the Study of Embryology 2. Retrieved from

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