MUSEUM OF COMPARATIVE ZOOLOGY. 281 



body cavity. The structure resembled the external glomerulus of Birds, 

 and he was inclined to interpret it as such in this case. 



In the preceding pages T have endeavored to present a comprehensive 

 resume of the development of the pronephros as described in groups of 

 Vertebrates other than Amphibia. In this review it has been shown 

 that an equivalent of the Amphibian pronephros has been claimed to 

 exist in all Craniota; and that a mode of development similar to that 

 described in the early part of the present paper has been found in 

 Selachii by Ruckert ('iS8) and van Wijhe ('89), in Petromyzon by 

 Knpfler ('88), and in Lepidosteus by Beard ('89). The Reptilian 

 pronephros as described by Hoffmann ('89), and that of the Chick 

 according to the account of Felix ('90), do not seem to me to be in per- 

 fect accord with this mode of development. 



It now remains for me to compare the results of my studies, as detailed 

 in the descriptive part of the present paper, with those which have been 

 recorded by other writers on the development of the Amphibian proneph- 

 ros. According to the account which at present receives most general 

 acceptance, the pronephros first appears as an outfolding of the somato- 

 pleure in the form of a longitudinal groove. The anterior end of this 

 groove is destined to become the pronephros ; the remaining portion is 

 constricted off to form the segmental duct. Since the process of con- 

 striction advances from before backwards, stages may be found in which 

 a completed tube is continuous posteriorly with a mere groove of the 

 somatopleure. In the anterior region, the groove remains in communica- 

 tion with the body cavity, and grows down towards the ventral surface 

 of the embryo in the form of a broad pocket. The slit-like peritoneal 

 opening of this pouch closes throughout the greater part of its length, 

 leaving, however, two or three regions of incomplete closure, the funda- 

 ments of the nephrostomes. The nephrostomal tubules are formed by the 

 fusion of the walls of the pouch between two nephrostomes. The regions of 

 fusion extend in vertical lines from the nephrostomal margin of the pouch 

 nearly to its ventral border, where there is left an unfused and therefore 

 continuous longitudinal tract constituting the canal which I have called 

 the collecting trunk. This view of the development of the pronephros, 

 although suggested by Willi. Midler (75), was first described in detail 

 by Goette ('75) for Bombinator, and was later extended to other Am- 

 phibia by the researches of Furbringer ('77). It has been entirely con- 

 firmed by Wichmann ('84), by Hoffmann ('86), and still more recently 

 by Marshall and Bles ('90 a ). 



