198 OUTLINES OF CHORD ATE DEVELOPMENT 



stome, which is then closed, and the nephrostomes thus cut off 

 from the body cavity (Fig. 73, C). The glomus shrinks, and 

 by the time of metamorphosis only a few scattered traces of 

 the pronephros remain, although the glomus remains indicated 

 for some months after metamorphosis. The pronephric ducts 

 do not take part in this process of degeneration, posterior to 

 the pronephric region; they remain, closed anteriorly. The dis- 

 appearance of the pronephros is correlated with the develop- 

 ment of the second excretory system, the mesonephros, to the 

 formation of which we shall now turn. 



B. THE MESONEPHROS OR WOLFFIAN BODY 



This begins to develop in tadpoles of 8-10 mm. Its rudiment 

 is formed by the nephrotomes of the seventh to twelfth somites, 

 and it is consequently both somatic and splanchnic in origin. 

 The nephrotomes of these segments fuse into a continuous 

 longitudinal strip of irregularly arranged cells, lying between 

 the pronephric duct and the dorsal aorta, along the posterior 

 cardinal vein. In this mass, cell groupings appear, forming 

 definite swellings of the cord. These are the rudiments of the 

 mesonephric vesicles; they are not strictly metameric, but are 

 somewhat more numerous than the mesodermal segments. 



All of these rudiments have essentially the same history 

 (Hall). First each becomes divided into a large ventral 

 chamber and a small dorsal one; the larger chamber is a primary 

 mesonephric unit, the smaller a secondary mesonephric unit (Fig. 

 73, B). The secondary units divide similarly, though much 

 later, into secondary units proper and tertiary mesonephric 

 units. All three series of units develop similarly though suc- 

 cessively, and we shall therefore describe only the history of 

 the primary series. 



From the vesicular primary unit two outgrowths are formed 

 (Fig. 74). One, known as the inner tubule, extends dorso- 

 laterally to the pronephric duct and opens into it. The 

 other, the outer tubule, grows ventro-medially to the peritoneum 

 with which it fuses and opens into the body cavity. The 



