(FIGURES 1.94 AND 1.95. Transverse section of the primitive-streak (primitive mouth) of the chick. Figure 1.94 a few hours after the commencement of incubation, Figure 1.95 a little later. (From Waldeyer.) h horn-plate, n nerve-plate, m skin-fibre-layer, f gut-fibre-layer, d gut-gland-layer, y primitive streak or axial plate, in which all four germinal layers meet, x structure of the chorda, u region of the later primitive kidneys.) Coelomation takes place in the vertebrates in just the same way as in the birds and reptiles. This was to be expected, as the characteristic gastrulation of the mammal has descended from that of the reptiles. In both cases a discoid gastrula with primitive streak arises from the segmented ovum, a two-layered germinal disk with long and small hinder primitive mouth. Here again the two primary germinal layers are only directly connected (Figure 1.96 pr) along the primitive streak (at the folding-point of the blastula), and from this spot (the border of the primitive mouth) the middle germinal layers (mk) grow out to right and left between the preceding. In the fine illustration of the coelomula of the rabbit which Van Beneden has given us (Figure 1.96) one can clearly see that each of the four secondary germinal layers consists of a single stratum of cells. Finally, we must point out, as a fact of the utmost importance for our anthropogeny and of great general interest, that the four-layered coelomula of man has just the same construction as that of the rabbit (Figure 1.96). A vertical section that Count Spee made through the primitive mouth or streak of a very young human germinal disk (Figure 1.97) clearly shows that here again the four secondary germ-layers are inseparably connected only at the primitive streak, and that here also the two flattened coelom-pouches (mk) extend outwards to right and left from the primitive mouth between the outer and inner germinal layers. In this case, too, the middle germinal layer consists from the first of two separate strata of cells, the parietal (mp) and visceral (mv) mesoblasts. (FIGURE 1.96. Transverse section of the primitive groove (or primitive mouth) of a rabbit. (From Van Beneden.) pr primitive mouth, ul lips of same (primitive lips), ak and ik outer and inner germinal layers, mk middle germinal layer, mp parietal layer, mv visceral layer of the mesoderm. FIGURE 1.97. Transverse section of the primitive mouth (or groove) of a human embryo (at the coelomula stage). (From Count Spee.) pr primitive mouth, ul lips of same (primitive folds), ak and ik outer and inner germinal layers, mk middle layer, mp parietal layer, mv visceral layer of the mesoblasts.)