2 9 



CLASSIFICATION OF VERTEBRATES. 



hind the yolk stalk into the extra embryonic part of the coelom 

 (Fig. 288), carrying with it the allantoic artery and the umbilical 

 vein or veins. Distally it expands into a large sac (which re- 

 ceives the excretion of the primitive kidneys), the outer surface 

 of the sac fusing with the chorion. The result of this, in ovipa- 

 rous forms, is that the allantoic structures come to lie close 



FIG. 289. Diagram of embryonic circulation in an amniote, the amnion 

 omitted for clearness. A, allantois ; A A, allantoic artery; C, carotids ; CA, caudal 

 artery; CV, caudal vein; DA, dorsal aorta ; DC, ductus Curvierii ; //, heart; If A, 

 hypogastric artery; Z, liver; OA, Ofy omphalomesaraic artery and vein; UV, 

 umbilical vein; V, vent; W, vitelline vein. The outline of the alimentary canal 

 blocked. 



beneath the shell, and hence, with their rich blood-supply, they 

 form efficient organs of foetal respiration. In the higher mam- 

 mals this allantois enters into close connection with the uterine 

 walls, thus giving rise to a structure both nutrient and indirectly 

 respiratory in character, the placenta, the featur^ of which will 

 be described in connection with that group. ; 



Basi- and presphenoid bones are present and a parasphenoid 

 occurs only in some reptiles (ophidia) as ' a small plate. The 



