MUSEUM OF COMPARATIVE ZOOLOGY. 285 



Of the two nephrostomes belonging to any pair, one opens freely into the 

 body cavity, the other communicates with a pronephric chamber, which 

 contains the glomus and is completely shut off from the body cavity. The 

 meaning of this condition I shall consider in the subsequent discussion. 

 The pairs of nephrostomes on each side are slightly more numerous than 

 the overlying protovertebrte. 



The origin of the so-called ventral part (conmion trunk) of the pro- 

 nephros has recently become the subject of controversy. According to 

 Goette the duct at first communicates with the posterior end of the widely 

 open pronephric pouch. At the same time that the nephrostomal canals 

 are formed by local fusions of the walls of the pouch, a similar process 

 constricts off the posterior ventral portion of the pouch ; this has the 

 effect of lengthening the duct, so that the point of its attachment is 

 carried forward to the place where the converging nephrostomal tubes 

 unite. The portion of the longitudinal canal in front of the most poste- 

 rior nephrostome represents the " ventral part " of the pronephros. 



According to Fiirbringer, the longitudinal groove which forms the 

 earliest fundament of the pronephros and duct becomes entii*ely con- 

 stricted off from the somatopleure as far forward as the opening which 

 leads into the pronephric pouch ; this slit-like opening then elongates 

 posteriorly, so as to extend into the region formerly occupied by the 

 longitudinal canal alone ; the latter thus comes to lie ventral to the last 

 nephrostomal canal, and forms the ventral part of the pronephros. 



Kellogg ('90) opposes the accounts of previous observers, and claims 

 that the ventral part " is formed from the ventral side of the dorsal part 

 of the pronephros, and anterior to the last nephrostome." Marshall and 

 Bles, alluding to Kellogg's description, declare that it is in exact ac- 

 cordance with the accounts of Goette and Fiirbringer. I have not been 

 able to satisfy myself as to the precise manner by which Kellogg con- 

 ceives the formation of the ventral part to have taken place ; but I think 

 he has said enough to contrast his position strongly with that of Fiir- 

 bringer, according to whom the ventral part of the pronephros first ap- 

 pears as a portion of the somatopleural fold immediately posterior to the 

 part which gives rise to the nephrostomal canals. Kellogg argues, how- 

 ever, that, were the views of previous authors correct, some portion of 

 the pronephros would appear behind the last nephrostome ; but this is 

 actually never the case. The force of this argument I am wholly unable 

 to appreciate, and I must in consequence feel some doubt as to whether 

 I have properly interpreted Kellogg's previous statements. 



According to Mollier, the " ventral part " is differentiated in the 



