. Chordate anatomy. Chordata; Anatomy, Comparative. 8o CHORDATE ANATOMY take form, a constriction involving both somatopleure and splanchno- pleure (Figs. 72, 74) appears between the yolk-sac and the remainder of the embryo. The constriction deepens until the embryo presents the appearance of a small animal having a narrow-necked globular sac sus- pended from the under side of the body (Figs. 72Z), 73). In amniotes the amnion is concerned in this constriction (Fig. 74). As the embryo increases in size the shrinking yolk-sac is drawn up into the body. The inner wall (splanchnopleure) of the sac
. Chordate anatomy. Chordata; Anatomy, Comparative. 8o CHORDATE ANATOMY take form, a constriction involving both somatopleure and splanchno- pleure (Figs. 72, 74) appears between the yolk-sac and the remainder of the embryo. The constriction deepens until the embryo presents the appearance of a small animal having a narrow-necked globular sac sus- pended from the under side of the body (Figs. 72Z), 73). In amniotes the amnion is concerned in this constriction (Fig. 74). As the embryo increases in size the shrinking yolk-sac is drawn up into the body. The inner wall (splanchnopleure) of the sac finally constitutes a small region of the wall of the intestine. In elasmobranchs the somatopleure of the yolk-sac finally flattens out and persists as a part of the abdominal wall. In reptiles and birds at the time of hatching the somatopleure is ruptured. Fig. -Young dogfish shortly before birth. The yolk-sac, containing a remnant of the yolk of the egg, protrvides from the ventral body-wall. at the constriction between the definitive body and the extra-embryonic structures and everything external to the rupture is abandoned. Embryonic and Fetal Membranes In the description (pages 33-36) of the reproductive arrangements in vertebrates a general account of the embryonic membranes, amnion, chorion and allantois, of reptiles, birds and mammals was given. The foregoing account of the origin of the germ layers and the shaping up of the embryonic body now makes it possible to appreciate the manner of formation of these membranes in terms of germ layers. All eggs are invested by protective coverings which are either pro- duced by the ovum itself or are secreted about the egg by the oviduct. Such membranes consist of material which is not cellular and not in any sense living. They have merely passive functions. The amnion, chorion and allantois are produced by the germ layers at a relatively advanced stage of the embryo. They are constituted of living cellular material and they
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