824 COMPARATIVE ANATOMY 



The eggs of different fishes range from large heavily-yolked eggs 

 with chitinous shells, as in the modern elasmobranchs, to the small 

 pelagic eggs of many modern teleosts. The eggs pass out through the 

 ducts of teleosts as mentioned in the preceding paragraph or through 

 abdominal pores as in ganoids and in some Physostomi. 



For the most part, fish-eggs are fertilized in the open water, although 

 there are many orders which practice internal impregnation and are 

 viviparous. 



Most of the teleosts are dioecious but some are hermaphroditic. 

 Serranus, a member of the Perch family is even self-impregnating, while 

 Chrysophrys is successively male and female, while cod and herring 

 often exhibit the hermaphrodite condition, though this is abnormal. 



Dogfish (Fig 462) : 



The pronephros is never functional as an excretory organ. The 

 nephrostomes fuse to form the ostium tubae in the female. 



The pronephric duct splits into both a Wolffian and a Miillerian 

 duct. The nephrostomes close in the adult. The anterior end of each 

 mesonephros is narrowed and in the male this connects with the anterior 

 end of the Wolffian duct to form a connection with the testes. The 

 epididymis consists of the coiled anterior end of this connection. 



The Miillerian ducts become known as oviducts. The oviducts of 

 both sides connect with the coelom. The common opening thus formed 

 is the ostium tubae abdominale. 



The eggs leave the ovary, pass to the ostium and are then carried 

 backward to a shell-gland. The enlarged portion of the tube forms the 

 uterus. 



In the male, the anterior end of the mesonephros forms the epididy- 

 mis while the vasa deferentia of both sides unite to form a urogenital 

 sinus. There is an oval sperm-sac connected on each side. Fertilization 

 is internal. 



The suprarenals are metameric and may be imbedded in the mesone- 

 phroi. 

 Amphibia (Fig. 457) : 



The pronephros functions until metamorphosis. The tubules then 

 degenerate. In the adult frog and other tailless amphibians the nephros- 

 tomes of the mesonephros separate from the nephridial tubules to join 

 with branches of the renal blood vessels so that the coelom is in direct 

 connection with the excretory system. 



The Wolffian duct carries the nephridial waste and the same duct 

 also acts as the vas deferens in the male just as it does in the dogfish. 

 Where these ducts enter the cloaca there is an enlargement on each to 

 form the seminal vesicle. 



The urinary bladder is ventral to the cloaca. The eggs pass into the 

 body cavity and thence into the ostium tubae. 



Fertilization is external in the tailless amphibians but internal in 

 tailed amphibians. The male of the tailed amphibians secretes a sub- 



