426 CHORD ATA. 



and the pancreas arises from several dorsal processes in the 

 same region. The essential epithelium of the gland in each 

 case arises in this way, the bulk of the organ being composed 

 of mesoblastic connective tissue and blood-vessels. 



Urogenital organs. — The urinary organs show a suc- 

 cession in the group of three separate series — the pronephros^ 

 mesonephros and metanephros. 



The pronephros is always situated far forward in the 

 coelom. It is functional in Myxine and in the tadpole of the 

 frog. It consists typically of three or more paired tubules 

 opening by funnels into the coelom and leading to the 

 exterior by a paired lateral pronephric duct. The meso- 

 nephros arises behind the pronephros and replaces it in de- 

 velopment. It is formed of a number of tubules arising 

 from the coelom and becoming connected with the pro- 

 nephric duct. The duct then splits into two, one of which 

 remains functional in the female as the Mtillerian duct or 

 oviduct, and the other becomes the Wolffian or mesonephric 

 duct, functioning in the female as a ureter, in the male as a 

 ureter and as a vas deferens. It is enabled to do this by 

 certain of the mesonephric tubules growing out towards the 

 testes, becoming connected with them and forming the vasa 

 efferentia. The other mesonephric funnels close in adult 

 life. This condition is found in the frog. 



In the skate and in Amniota the metanephros arises as a 

 set of tubules posterior to the mesonephros. They become 

 connected to the cloaca by ureters, and the mesonephros 

 then atrophies so far as the excretory function is concerned. 

 It persists in the male rabbit as the epididymis. In the 

 metanephros the tubules have no funnels. The exact 

 meaning of this successive replacement of one kind of 

 excretory organ by another throughout the sub-phylum is 

 unknown. 



Development. — The types of development already out- 

 lined are very diverse, but it is possible to trace a phyletic 

 sequence from one to the other. 



In young forms, even more than in adults, because the 

 reproductive element is not present, the nutritive conditions 

 are the secret of the structural modifications, and we can 

 discern in the vertebrate series no less than five different 



