The most suitable and most available objects of study in this class are the eggs of our indigenous amphibia, the tailless frogs and toads, and the tailed salamander. In spring they are to be found in clusters in every pond, and careful examination of the ova with a lens is sufficient to show at least the external features of the segmentation. In order to understand the whole process rightly and follow the formation of the germinal layers and the gastrula, the ova of the frog and salamander must be carefully hardened; then the thinnest possible sections must be made of the hardened ova with the microtome, and the tinted sections must be very closely compared under a powerful microscope.
The ova of the frog or toad are globular in shape, about the twelfth of an inch in diameter, and are clustered in jelly-like masses, which are lumped together in the case of the frog, but form long strings in the case of the toad. When we examine the opaque, grey, brown, or blackish ova closely, we find that the upper half is darker than the lower. The middle of the upper half is in many species black, while the middle of the lower half is white.* (* The colouring of the eggs of the amphibia is caused by the accumulation of dark-colouring matter at the animal pole of the ovum. In consequence of this, the animal cells of the ectoderm are darker than the vegetal cells of the entoderm. We find the reverse of this in the case of most animals, the protoplasm of the entoderm cells being usually darker and coarser-grained.) In this way we get a definite axis of the ovum with two poles. To give a clear idea of the segmentation of this ovum, it is best to compare it with a globe, on the surface of which are marked the various parallels of longitude and latitude. The superficial dividing lines between the different cells, which come from the repeated segmentation of the ovum, look like deep furrows on the surface, and hence the whole process has been given the name of furcation. In reality, however, this "furcation," which was formerly regarded as a very mysterious process, is nothing but the familiar, repeated cell-segmentation. Hence also the segmentation-cells which result from it are real cells.
(FIGURE 1.40. The cleavage of the frog's ovum (magnified ten times). A stem-cell. B the first two segmentation-cells. C four cells. D eight cells (4 animal and 4 vegetative). E twelve cells (8 animal and 4 vegetative). F sixteen cells (8 animal and 8 vegetative). G twenty-four cells (16 animal and 8 vegetative). H thirty-two cells. I forty-eight cells. K sixty-four cells. L ninety-six cells. M 160 cells (128 animal and 32 vegetative).
(FIGURES 1.41 TO 1.44. Four vertical sections of the fertilised ovum of the toad, in four successive stages of development. The letters have the same meaning throughout: F segmentation-cavity. D covering of same (D dorsal half of the embryo, P ventral half). P yelk-stopper (white round field at the lower pole). Z yelk-cells of the entoderm (Remak's "glandular embryo"). N primitive gut cavity (progaster or Rusconian alimentary cavity). The primitive mouth (prostoma) is closed by the yelk-stopper, P. s partition between the primitive gut cavity (N) and the segmentation cavity (F). k k apostrophe, section of the large circular lip-border of the primitive mouth (the Rusconian anus). The line of dots between k and k apostrophe indicates the earlier connection of the yelk-stopper (P) with the central mass of the yelk-cells (Z). In Figure 1.44 the ovum has turned 90 degrees, so that the back of the embryo is uppermost and the ventral side down. (From Stricker.)).
The unequal segmentation which we observe in the ovum of the amphibia has the special feature of beginning at the upper and darker pole (the north pole of the terrestrial globe in our illustration), and slowly advancing towards the lower and brighter pole (the south pole). Also the upper and darker hemisphere remains in this position throughout the course of the segmentation, and its cells multiply much more briskly. Hence the cells of the lower hemisphere are found to be larger and less numerous. The cleavage of the stem-cell (Figure 1.40 A) begins with the formation of a complete furrow, which starts from the north pole and reaches to the south (B). An hour later a second furrow arises in the same way, and this cuts the first at a right angle (Figure 1.40 C). The ovum is thus divided into four equal parts. Each of these four "segmentation cells" has an upper and darker and a lower, brighter half. A few hours later a third furrow appears, vertically to the first two (Figure 1.40 D). The globular germ now consists of eight cells, four smaller ones above (northern) and four larger ones below (southern). Next, each of the four upper ones divides into two halves by a cleavage beginning from the north pole, so that we now have eight above and four below (Figure 1.40 E). Later, the four new longitudinal divisions extend gradually to the lower cells, and the number rises from twelve to sixteen (F). Then a second circular furrow appears, parallel to the first, and nearer to the north pole, so that we may compare it to the north polar circle. In this way we get twenty-four segmentation-cells—sixteen upper, smaller, and darker ones, and eight smaller and brighter ones below (G). Soon, however, the latter also sub-divide into sixteen, a third or "meridian of latitude" appearing, this time in the southern hemisphere: this makes thirty-two cells altogether (H). Then eight new longitudinal lines are formed at the north pole, and these proceed to divide, first the darker cells above and afterwards the lighter southern cells, and finally reach the south pole. In this way we get in succession forty, forty-eight, fifty-six, and at last sixty-four cells (I, K). In the meantime, the two hemispheres differ more and more from each other. Whereas the sluggish lower hemisphere long remains at thirty-two cells, the lively northern hemisphere briskly sub-divides twice, producing first sixty-four and then 128 cells (L, M). Thus we reach a stage in which we count on the surface of the ovum 128 small cells in the upper half and thirty-two large ones in the lower half, or 160 altogether. The dissimilarity of the two halves increases: while the northern breaks up into a great number of small cells, the southern consists of a much smaller number of larger cells. Finally, the dark cells of the upper half grow almost over the surface of the ovum, leaving only a small circular spot at the south pole, where the large and clear cells of the lower half are visible. This white region at the south pole corresponds, as we shall see afterwards, to the primitive mouth of the gastrula. The whole mass of the inner and larger and clearer cells (including the white polar region) belongs to the entoderm or ventral layer. The outer envelope of dark smaller cells forms the ectoderm or skin-layer.
In the meantime, a large cavity, full of fluid, has been formed within the globular body—the segmentation-cavity or embryonic cavity (blastocoel, Figures 1.41 to 1.44 F). It extends considerably as the cleavage proceeds, and afterwards assumes an almost semi-circular form (Figure 1.41 F). The frog-embryo now represents a modified embryonic vesicle or blastula, with hollow animal half and solid vegetal half.
Now a second, narrower but longer, cavity arises by a process of folding at the lower pole, and by the falling away from each other of the white entoderm-cells (Figures 1.41 to 1.44 N). This is the primitive gut-cavity or the gastric cavity of the gastrula, progaster or archenteron. It was first observed in the ovum of the amphibia by Rusconi, and so called the Rusconian cavity. The reason of its peculiar narrowness here is that it is, for the most part, full of yelk-cells of the entoderm. These also stop up the whole of the wide opening of the primitive mouth, and form what is known as the "yelk-stopper," which is seen freely at the white round spot at the south pole (P). Around it the ectoderm is much thicker, and forms the border of the primitive mouth, the most important part of the embryo (Figure 1.44 k, k apostrophe). Soon the primitive gut-cavity stretches further and further at the expense of the segmentation-cavity (F), until at last the latter disappears altogether. The two cavities are only separated by a thin partition (Figure 1.43 s). With the formation of the primitive gut our frog-embryo has reached the gastrula stage, though it is clear that this cenogenetic amphibian gastrula is very different from the real palingenetic gastrula we have considered (Figures 1.30 to 1.36).
In the growth of this hooded gastrula we cannot sharply mark off the various stages which we distinguish successively in the bell-gastrula as morula and gastrula. Nevertheless, it is not difficult to reduce the whole cenogenetic or disturbed development of this amphigastrula to the true palingenetic formation of the archigastrula of the amphioxus.
(FIGURE 1.45. Blastula of the water-salamander (Triton). fh segmentation-cavity, dz yelk-cells, rz border-zone. (From Hertwig.)
FIGURE 1.46. Embryonic vesicle of triton (blastula), outer view, with the transverse fold of the primitive mouth (u). (From Hertwig.)