. The development of the chick; an introduction to embryology. Birds -- Embryology. HEAD-FOLD TO TWELVE SOMITES 123 In the region of the area pellucida the entoderm is very thin; at its boundary it passes rather abruptly into the large rounded vesi- cular cells of the yolk-sac entoderm, which becomes continuous at the margin of the vascular area with the germ-wall; the latter continues to the periphery where it merges in the undifferen- tiated cell-mass (zone of junction) (Figs. 68 A-68 E). The large neural tube is not yet closed. Beneath the neural tube is a sec- tion of the solid rod-like no
. The development of the chick; an introduction to embryology. Birds -- Embryology. HEAD-FOLD TO TWELVE SOMITES 123 In the region of the area pellucida the entoderm is very thin; at its boundary it passes rather abruptly into the large rounded vesi- cular cells of the yolk-sac entoderm, which becomes continuous at the margin of the vascular area with the germ-wall; the latter continues to the periphery where it merges in the undifferen- tiated cell-mass (zone of junction) (Figs. 68 A-68 E). The large neural tube is not yet 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 13 s. Ectam., Ectamnion. F. B., Fore-brain. H. B., Hind-brain. Inf., In- fundibulum. M. B., Mid-brain, pr'c. pi., Precardial plate. T. p., Tuber- culum posterius. Other abbreviations as before. The mesoderm (Fig. 68 A, B, C) lies between the parts already named; it consists on each side of the middle line of the following parts: (1) the mesoblastic somite, a block of cells that radiate from a central cavity filled with irregularly disposed cells; (2) the intermediate cell-mass or nephrotome, forming a narrow connect- ing bridge between the somite and the lateral plate; (3) the lateral plate, split into two layers, external, known as the somatic layer, and internal or splanchnic layer. The cavity between the two layers is the ccelome or body-cavity; it is very narrow next the nephrotome, but widens as it extends laterally to the margin of the vascular area, and is divided by various strands of cells extending from somatic to splanchnic layers, thus indicating its origin by fusion of coelomic vesicles. The ectoderm plus the somatic layer constitute the somato- pleure, from which the body-wall, amnion, and chorion are derived, and the entoderm plus the splanchnic layer form the splanchno-. Please note that these images are extracted from scanned page images that may have been digitally enhanced for read
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