FCETAL MEMBRANES. 105 



same time, as already noted (p. 102), the growth of the amnion ventrally results 

 in a sharp constriction which separates the embryo from the underlying yolk. 

 This constriction is emphasized by constant lengthwise growth of the embryo. 

 Following the gradual growth of the entoderm around the yolk, the mesoderm 

 also gradually extends around, at the same time splitting into visceral and 

 parietal layers, so that the entoderm is closely invested by visceral mesoderm 

 (Fig. 97, a, b, c and d). Finally, both entoderm and mesoderm enclose com- 

 pletely the mass of yolk. The yolk thus becomes enclosed in the yolk sac 

 which consists of two layers, entoderm and visceral mesoderm. The constricted 

 connection between the yolk sac and the embryo is the yolk stalk. It is seen by 

 reference to the diagrams (Fig. 97) that the entoderm lining the yolk sac is 



Fig. 98. — Diagrammatic longitudinal section of selachian embryo. Herlwig. 

 a., Anus; d., yolk sac; in., intestinal umbilicus; ds., visceral layer of yolk sac; hs., parietal layer of 

 yolk sac; kn., dermal umbilicus; Ik*, ccelom; lh 2 , exocoelom; m., mouth; St., yolk stalk. 



directly continuous through the yolk stalk with the entoderm lining the primi- 

 tive gut. The transition line between extra- and intraembryonic entoderm is 

 sometimes referred to as the intestinal umbilicus, in contradistinction to the line 

 of union, on the outside of the yolk stalk, of amniotic and embryonic ecto- 

 derm (the latter becoming later the epidermis) which is known as the dermal 

 umbilicus. 



As in Fishes and Amphibians, so also in Reptiles and Birds, the yolk furnishes 

 nourishment for the growing embryo, and is conveyed to the embryo by 

 the blood. At a very early stage the mesoderm layer of the yolk sac (visceral 

 mesoderm) becomes extremely vascular. This vascular area is indicated by an 

 irregularly reticulated appearance in the periphery of the blastoderm and is 

 known as the area vasculosa (Fig. 74). The area vasculosa increases in size as 

 the mesoderm grows around the yolk and its vessels become continuous with 

 those in the embryo (Fig. 212). Some of these vessels enlarge as branches of 

 two large vessels which are given off from the primitive aortae, the vitelline or 

 omphalomesenteric arteries. (When the two aortas fuse to form a single 

 vessel, the proximal ends of the vitelline arteries' fuse likewise.) The branches 

 of the arteries ramify in the mesoderm over the surface of the yolk and then 



