. A laboratory manual and text-book of embryology. Embryology. CHICK EMBRYO OF TWENTY-SEVEN SEGMENTS 75 developing. The head-fold of the somatopleure forms first and envelops the head, the tail-fold makes its appearance later. The two folds extend laterad, meet and fuse (Fig. 67 B). The inner leaf of the folds forms the amnion, the remainder of the extra-embryonic somatopleure becomes the chorion. The actual appear- ance of these structures and their relation to the embryo we have seen in Figs. 60 and 61. The amnion, with its ectodermal layer inside, completely surrounds the embryo by the four
. A laboratory manual and text-book of embryology. Embryology. CHICK EMBRYO OF TWENTY-SEVEN SEGMENTS 75 developing. The head-fold of the somatopleure forms first and envelops the head, the tail-fold makes its appearance later. The two folds extend laterad, meet and fuse (Fig. 67 B). The inner leaf of the folds forms the amnion, the remainder of the extra-embryonic somatopleure becomes the chorion. The actual appear- ance of these structures and their relation to the embryo we have seen in Figs. 60 and 61. The amnion, with its ectodermal layer inside, completely surrounds the embryo by the fourth day, enclosing a cavity filled with amniotic fluid (Fig. 68). In this the embryo floats and is thus protected from injury. The chorion is of little importance to the chick. It is at first incomplete but eventually entirely surrounds the embryo and its other appendages. Yolk-sac and Yolk-stalk.—While the amnion and chorion are developing during the second and third day, the embryo grows rapidly. 4gy- -~^Af. jm The head- and tail-folds elon- gate and the trunk expands lat- erally until only a relatively nar- row stalk of the splanchno- pleure connects the embryo with the yolk. This portion of the splanchnopleure has grown more slowly than the body of the embryo and is termed the yolk-stalk. It is continuous with the splanchnopleure which envelops the yolk and forms the yolk-sac. The process of un- equal growth by which the em- bryo becomes separated from the yolk has been described as a process of constriction. This, as Minot points out, is an error. The splanchno- pleure at first forms only an oval plate on the surface-of the yolk but eventually encloses it. In Fig. 67, C and D, the relation of the embryo to the yolk-sac is seen at the end of the first week of incubation. The vitelline vessels ramify on the surface of the yolk-sac and through them all the food material of the yolk is conveyed to the chick during the incubation period (about twenty-one days). Allantois.—We
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectembryology, bookyear1
