Development of Excretory Organs. 139 



of these is the vitelline, which goes to the yolk sac ; the second 

 supplies the allantois, and is known as the allantoic. 



The veins which return the blood to the heart are to begin 

 with, and partly, arranged after the fashion of those of a fish. 

 There are, as in those vertebrates, an anterior and a posterior 

 pair of cardinal veins, which unite to form a pair of Cuvierian 

 veins, into which blood is poured from the anterior and pos- 

 terior regions of the body. Before these have appeared 

 two large vitelline veins unite to enter the heart by a single 



Fig. 72. — Surface view of Chick at end of third day, to illustrate vitelline veins. 

 (From Marshall.) 



SM, vitelline membrane ; ad, area pellucida ; Av, area vasentosa, with ramifying 

 vitelline veins ; ak, area space ; em, embryo. 



trunk, and a little later a pair of allantoic veins join them. 

 Still later a median vena cava posterior, the persistent vein 

 of the posterior part of the body in the adult, opens through 

 the same trunk, known as the meatus venosus. The anterior 

 cardinals persist as the jugular veins, and the Cuvierian sinuses 

 as the vense cavse anteriores. 



The Excretory Organs. — In order to properly understand 

 the excretory system, it will be necessary to supplement the 

 account of the development of this system in the chick with 

 some account of what occurs in the tadpole. In the latter the 

 first part of the excretory system to appear is a rod of meso- 

 blast, at first solid, afterwards hollow, which opens to the 

 exterior vid, the cloaca, and in front communicates with the 



