Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/180

130 embryo be viewed in transverse section (Fig. 175), this deepening is seen to be produced through the rising up of the External blastodermic membrane (Fig. 175, a) in two heaps, called Laminae Dorsales (Fig. 175, L), which, growing towards each other, finally coalesce, thus converting the Primitive groove into a tube (Fig. 176, K). This tube is the rudimentary central nervous system. Directly underneath this tube, in the Middle membrane, however, is seen a cylindrical rod of cells, the Chorda Dorsalis (Fig. 176, v), in which are developed the bodies of the vertebrae (segments of spine). By looking at Figs. 169 to 172 (Dog or Man), we see how, by a continually constructing process, the upper portion of the Internal blastodermic membrane (Fig. 169, I), with that part of the Middle membrane lying upon it, is gradually pinched off from the lower (Fig. 169, a), until, finally, only a narrow pedicle connects the two. The upper pinched-off portion is the primitive alimentary canal (Fig. 170, I), the lower the umbilical vesicle, or yolk-bag. The umbilical vesicle (Figs. 169 to 172, a), in the course of development, passes away, the time of its disappearance varying in different animals: thus, in the Trout it is retained till the sixtieth day. By referring to Figs. 169, 172, it will be seen that the alimentary canal and umbilical vesicle are composed of two layers. The inner layer, or the Internal blastodermic membrane, develops the epithelium of the mucous membrane; the outer layer, or the lower half of the Middle membrane, makes the wall of the alimentary canal. This is a very important distinction, since the lower lungs, etc., which first appear as buds sprouting from the alimentary canal, exhibit the same structure. In the Batrachia (Frog), and some Fishes, however, the whole of the Internal blastodermic membrane, with that part of the Middle membrane lying upon it, is used up in the formation of the alimentary canal, which is developed in a different manner from that of the dog or man; there is, therefore, no