500 URO-GEN1TAL SYSTEM. 



tion, which is more durable than in the ovipara, which has 

 its source from an organ called the umbilical vesicle ; finally, 

 when the body of the embryo is developed it is protected in 







Fig. 137. Commencement and development of the egg.* 



a pouch or diverticulum, amnion^ whose fluid contents ward 

 off sudden compressions. 



Umbilical vesicle. When the blasted erm (see p. 17) is 

 formed around the egg, by its simple nutrition, as above 

 shown, it attains a considerable size, from the fact that its 

 interior forms a cavity, whilst the division of the blastoderm 

 into three folds (u, a) becomes more pronounced at that 

 place in which the body of the embryo will be formed (Fig. 

 138). But as the embryo becomes gradually developed, the 

 circular region, by which it forms a part of the common 

 blastodermic vesicle, gradually recedes (from 9 to al, Fig. 

 139) in such a manner, that soon the primary cavity becomes 

 divided into two secondary cavities (Fig. 137, o and 12), one 

 of which forms a portion of the embryo (12), its future 

 intestinal cavity (see pp. 184, 232, and 424), and the other 

 forms a vesicle placed above the ventral portion of the 

 embryo (Fig. 137, o), the umbilical vesicle, which communi- 

 cates with the intestine only by a canal called the omphalo- 

 mesenteric duct (Fig. 139 and 140) ; the place at which this 



* 1, Vitelline membrane. 2, External layer of the blastoderm. 3, Middle 

 layer. 4, Internal layer of the blastoderm. 5, Form of the embryo. 6, Cephalic 

 fold of the amnion. 7, Caudal fold of the amnion. 8, Extremity of the ce- 

 phalic fold, which tends to join the corresponding extremity of the caudal fold. 

 9, Point for formation of heart. 10, Umbilical vesicle, yolk sac. 12, A portion 

 of the internal fold of the blastoderm, from which the intestine will be formed. 



