426 CHORDATA. 
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 
ceelom. It is functional in AZyxine and in the tadpole of the 
frog. It consists typically of three or more paired tubules 
opening by funnels into the ccelom and leading to the 
exterlor 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 ccelom and becoming connected with the pro- 
nephric duct. The duct then splits into two, one of which 
remains functional in the female as the Afi//erian 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 
oferentia. The other mesonephric funnels close in adult 
life. This condition is found in the frog. 
In the skate and in Ammniota the mefanephros 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 epzdidymis. 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 
