ANATOMY. 481 



coursing backwards gives off branches to the trunk and 

 then divides into arteries for the yolk-sac and allantois 

 respectively. The veins meantime have developed. These 

 embryonic veins are very different from those of the later 

 bird. They consist of a pair of anterior and a pair of posterior 

 cardinal veins like those of a dog-fish, and again, like the fish, 

 these four veins open into a paired ' ductus Cuvieri ' before 

 reaching the sinus vencsus. There is also a median vein 

 which receives blcod from the yolk-sac and allantois, and forms 

 the ' ductus venosus,' which thus returns blood, aerated and 

 laden with yolk, direct to the heart. 



cc. The Muscles, Skeleton and Connective Tissues. 



These arise mainly from the ' protovertebrse ' which 

 increase in number from before backwards. About thirty- 

 six are present at the end of the third day. The full number 

 is attained on the sixth day. Their relations are as follows : 



1-4. Occipital. 



5-16. Cervical. 

 17-19. Brachial. 

 20-25. Abdominal. 

 26-32. Pelvic. 

 33-35. Cloacal. 

 36-52. Caudal. 



dd. The Renal Organs. 



The kidney of vertebrates shows a division into three 

 sets of structures (see p. 262), called respectively pronephros, 

 mesonephros, and metanephros. 



The pronephros is the larval kidney, and as few vertebrates 

 pass through a larval stage in their life-history, this structure 

 is very imperfect. 



The mesonephros is the functional kidney of the adult 

 fish and amphibian and the embryonic functional kidney of 

 higher forms. Accordingly it is always well developed, but 

 for a varying time. 



The metanephros is the functional kidney of adult birds, 



