EXCRETORY AND REPRODUCTIVE SYSTEMS 349 



is the functional larval kidney in the lower vertebrates, and the 

 pronephric duct grows back to the cloaca without waiting for 

 the mesonephric tubules to develop. When these form, they 

 find the pronephric duct ready-made to receive them. After 

 receiving the mesonephric tubules the pronephric duct becomes 

 known as the mesonephric duct, and the pronephros 

 degenerates (except in the bony fish Fierasfer and Gobiesox). 

 The functional kidney in the adult fish or amphibian is the 

 mesonephros. 



The cavity of each tubule (pronephric or mesonephric) 

 becomes shut off from the splanchnocoel, although the opening 

 of the tubule into the splanchnocoel (the ciliated funnel or 

 ccelomostome) may persist (as in Selachians and amphibia) 

 on the median side of the occlusion. The cavity of the tubule 

 now becomes known as a Bowman's capsule, and its wall is 

 indented by capillaries from the dorsal aorta and leading to the 

 posterior cardinal vein, forming the glomerulus. Bowman's 

 capsule and the glomerulus together form a Malpighian 

 corpuscle. Primitively, these corpuscles are segmen tally 

 arranged, and this condition is retained in Myxine. In other 

 forms the number of Malpighian corpuscles is greatly increased 

 by the formation of others by budding. 



In the Cyclostomes, the germ-cells in the two sexes are 

 shed into the coelomic cavity, and make their way to the exterior 

 by a pair of pores at the base of the mesonephric ducts. In 

 all higher forms the sperms are never shed into the ccelom, 

 but led by vasa efferentia to the vas deferens, primitively 

 passing through the tubules of the mesonephros. The vasa 

 efferentia are the ccelomic funnels leading into the tubules, 

 and the mesonephric duct forms the vas deferens or Wolffian 

 duct. In addition, on each side there is another duct, in the 

 embryo. This is the Miillerian duct or oviduct, which develops 

 in the females but becomes reduced in the males. The 

 Miillerian duct in the Selachians arises by splitting off from the 

 Wolffian duct, but in other forms it grows back independently 

 from its opening into the ccelom (the oviducal funnel or 

 Fallopian tube) in front, to the cloaca behind. The eggs then 

 are shed into the ccelom whence they enter the oviducts, 



