. The development of the chick; an introduction to embryology . e entire blastoderm at the stage of 10 s,through the sixth somite (Fig. 68), shows the following parts: The ectoderm bounds the section above; it is thickened in theangle between the neural tube and the somites, and becomesthinner as it is traced peripherally; at the extreme periphery ofthe blastoderm it merges into a mass of cells that interpenetratethe yolk. Ventrally the boundary of the section is formed bythe entoderm which is slightly arched upwards in the middle line. HEAD-FOLD TO TWELVE SO:\IITES 123 In the region of the ar
. The development of the chick; an introduction to embryology . e entire blastoderm at the stage of 10 s,through the sixth somite (Fig. 68), shows the following parts: The ectoderm bounds the section above; it is thickened in theangle between the neural tube and the somites, and becomesthinner as it is traced peripherally; at the extreme periphery ofthe blastoderm it merges into a mass of cells that interpenetratethe yolk. Ventrally the boundary of the section is formed bythe entoderm which is slightly arched upwards in the middle line. HEAD-FOLD TO TWELVE SO:\IITES 123 In the region of the area pellucida the entoderm is very thin; atits boundary it passes rather abruptly into the large rounded vesi-cular cells of the yolk-sac entoderm, which becomes continuousat the margin of the vascular area with the germ-wall; thelatter continues to the periphery where it merges in the undifferen-tiated cell-mass (zone of junction) (Figs. 68 A-68 E). The largeneural tube is not vet closed. Beneath the neural tube is a sec-tion of the solid rod-like Fig. 67. — Median longitudinal section of the head of an embryo of s. Ectam., Ectamnion. F. B., Fore-brain. H. B., Hind-brain. Inf., In-fundibulum. M. B., Mid-brain, prc. pi., Precardial plate. T. p., Tuber-culum posterius. Other abbreviations as before. The mesoderm (Fig. 68 A, B, C) lies between the parts alreadynamed; it consists on each side of the middle line of the followingparts: (1) the mesohlastic somite, a block of cells that radiatefrom a central cavity filled with irregularly disposed cells; (2) theintermediate cell-mass or nephrotome, forming a narrow connect-ing bridge between the somite and the lateral plate; (3) thelateral plate, split into two layers, external, known as the somaticlayer, and internal or splanchnic layer. The cavity between thetwo layers is the coelome or hody-cavity; it is very narrow next thenephrotome, but widens as it extends laterally to the marginof the vascular area, and is di
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