. The development of the chick; an introduction to embryology . extra-embryonic. From itarise the embryonic membranes known as the amnion, chorion,and yolk-sac. The allantois (Fig. 33 B) arises as an outgrowthfrom the hind-gut of the embryo, and spreads within the extra-embryonic body-cavity; it thus becomes an extra-embryonicmembrane secondarily. The growth of the embryo and of theextra-embryonic blastoderm are distinct, though interdependent,processes going on at the same time. During the first four days of development the blastodermspreads very rapidly (Figs. 32 and 33). Thus on the fourth
. The development of the chick; an introduction to embryology . extra-embryonic. From itarise the embryonic membranes known as the amnion, chorion,and yolk-sac. The allantois (Fig. 33 B) arises as an outgrowthfrom the hind-gut of the embryo, and spreads within the extra-embryonic body-cavity; it thus becomes an extra-embryonicmembrane secondarily. The growth of the embryo and of theextra-embryonic blastoderm are distinct, though interdependent,processes going on at the same time. During the first four days of development the blastodermspreads very rapidly (Figs. 32 and 33). Thus on the fourth day(Fig. 33 A) the greater portion of the yolk is already the overgrowth of the yolk proceeds much more slowly(cf. Fig. 33 B). In the opaque area there arise,as concentric zones,the area vasculosa distinguished by its blood-vessels and the area 61 62 THE DEVELOPxAIEXT OF THE CHICK vitellina, which may be divided into inner and outer zones(Figs. 32 and 33). The development of the embryo during thesame period is indicated in the same Fig. 32. — A. Hens egg at about the twenty-sixth hour of incubation, toshow the zones of the blastoderm and the orientation of the embryo withreference to the axis of the shell. (After Duval.)B. Yolk of hens egg incubated about 50 hours to show the extent ofovergrowth of the blastoderm. (After Duval.) A. C, Air chamber, a. p., Area pellucida. a. v., Area vasculosa. a. v. e.,Area vitellina externa, a. v. i., Area vitellina interna. Y., Uncoveredportion of yolk. The blastoderm early becomes divided in two layers as faras the margin of the vascular area. The outer layer, knownas the somatopleure, is continuous with the body-wall, which isopen ventrally in the young embryo. The inner one, known asthe splanchnopleure, is continuous with the wall of the intestinewhich is likewise open ventrally. The space between these twomembranes, the extra-embryonic body-cavity, is continuouswith the body-cavity of the embryo. Ultimately,
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