Section 29. the Nature of the Amnion Will Be Understood by Following

Figures 4b, 5, and 6 on Sheet 23. The three embryonic layers are indicated by broken lines, dots, and black lines, just as they are in the frog diagrams. Not only is the embryo slowly pinched off from the yolk sac (y.s.), but, as the yolk is absorbed beneath it, and it grows in size, it sinks into the space thus made, the extra-embryonic somatopleur and epiblast rise up round it as two folds, which are seen closing in 5, and closed in 6, over the dorsal side of the young chick. In this way a cavity, a., lined by epiblast, and called the amniotic cavity, is formed. Dorsal to this, in 6, comes a space lined by somatic mesoblast, and continuous with p.p., the pleuro-peritoneal cavity, or body cavity of the embryo. Outside this, again, is a layer, of somatopleur internally and epiblast externally, the false amnion (f.a.), which is continuous with the serous membrane (s.m.) enclosing the rest of the egg. The student should, carefully copy these diagrams, with coloured pencils or inks for the different layers, and should compare them with the more realistic renderings of Figures 2, 5, and 8, Sheet 24.