THE LATER DEVELOPMENT OF THE FROG 201 



This pare of the outer tubule now separates from the remainder, 

 retaining the connection with the inner tubule, while the distal 

 part retains its connection with the body cavity; this con- 

 nection now becomes ciliated and forms a typical nephrostome. 

 This nephrostomal region is short and effects a new connection 

 at its inner end, with the sinus of the posterior cardinal vein. 

 It will be remembered that, as the tubules of the mesonephros 

 enlarge (15 mm.) this body seems to extend freely into the 

 posterior cardinal vein, which has a sinusoidal character here 

 and interpenetrates the substance of the mesonephros, its walls 

 being closely reflected around the surfaces of the tubules. 



During the later development of the mesonephros, the sec- 

 ondary and tertiary units acquire, similarly, connections with 

 the mesonephric duct, Malpighian bodies, and nephrostomal 

 connections with the body cavity and cardinal vein. Addi- 

 tional outer tubules and nephrostomes are formed later, but it 

 is not clear whether they form as independent evaginations 

 of the peritoneum or by splitting off from those previously 

 formed; perhaps both processes occur. The number finally 

 formed is very large (about two hundred according to Marshall 

 and Bles.) 



Upon the development of the reproductive organs certain 

 of the mesonephric tubules become modified and take on new 

 functions; these processes will be described below. 



Just before metamorphosis the urinary bladder appears as a 

 median ventral evagination of the wall of the cloaca, nearly 

 opposite the openings of the mesonephric ducts or ureters. The 

 rudiment forms just at the border between the ectoderm and 

 endoderm lining the cloaca. It is at first a long narrow sac, 

 directed anteriorly; later its diameter increases and as it 

 enlarges it becomes bifid at its extremity. 



2. The Reproductive System 



In order to make the devlopment of this system easier to 

 understand we may first repeat, very briefly, the arrangement 

 and composition of the adult system as described in the pre- 



