FROM TWELVE TO THIRTY-SIX SOMITES 193 



homologized with these. On the other hand, they appear at the 

 same time as the first glomeruli of the mesonephros (q. v.) and 

 possess, by way of the intermediate tubules, undeniable resem- 

 blance to the latter. 



At the stage of 10 somites the pronephros is represented by a series 

 of thickenings of the somatic layer of the intermediate cell-mass extend- 

 ing from the fifth somite backward to the segmental plate. In an embryo 

 of 13 somites the connection between the somite and nephrotome is lost, 

 and the pronephric tubules from the ninth to the thirteenth somites 

 have united to form the beginning of the Wolffian duct. 



In an embryo of 16 somites a single pronephric tubule was found 

 at the level of the hind end of the fifth somite, and was very distinct 

 on one side but hardly discernible on the other. Its posterior continua- 

 tion was soon lost, and the next distinct tubules were between the ninth 

 and tenth somites; from here back there was a tubule opposite the hind 

 end of each somite to the fifteenth, which was the last, and the duct was 

 continuous. 



In an embryo of 21 somites, one finds only isolated remnants of the 

 pronephros in front of the eleventh somite; from here to the fifteenth 

 the tubules are well developed and retain their connection both with 

 the Wolffian duct and the lateral plate. The Wolffian duct extends 

 back of this place to the region of the posterior half of the segmental 

 plate. 



At the 35 s stage the pronephric tubules are much degenerated, 

 but the nephrostomes usually remain. In one embryo there was found a 

 well-developed pronephric tubule on each side in the thirteenth somite. 

 That of the left side had a wide nephrostome, the lumen of which stopped 

 short of the tubul ; the nephrostome of the right side was rudimentary. 

 On the right side the Wolffian duct extended no farther forward, but 

 on the left side it was continued to the eleventh somite, and rudimentary 

 pronephric strands uniting it to the coelomic epithelium existed in both 

 eleventh and twelfth somites. Here the Wolffian duct stopped. But 

 isolated pronephric rudiments and minute nephrostomes were found on 

 both sides as far forward as the tenth somite. 



The Wolffian Duct. The Wolffian duct consists according to 

 the foregoing account of two parts, (1) an anterior division formed 

 by the union of the pronephric tubules, and (2) a posterior divi- 

 sion that arises as an outgrowth of the anterior part. The latter 

 grows backward above the intermediate cell-mass as a solid 

 cord (Fig. 107), apparently by active multiplication of its own 

 cells, without participation of the neighboring mesoderm or 



