280 BULLETIN OF THE 



are all differentiated from the solid " Wolffian blastema," and never have 

 any connection with the body cavity. Posterior to the last pronephric 

 canal, 5-6 free glomeruli are to be found. The anterior canals form 

 much earlier than the posterior, indeed they wholly abort before the 

 mesonephros attains its final development; and they together with the 

 free glomeruli are, in his opinion, to be regarded as equivalent to 

 the pronephros and glomus of Amphibia. Mihalkovics also mentions 

 the occurrence of transitional glomeruli ; these are typical glomeruli 

 which lie near the peritoneal covering of the Wolffian body. It seems 

 to me probable that these glomeruli really belong to the mesonephros, 

 and that at least a portion of the "external glomeruli" belong in reality 

 to the class which 1 have designated transitional glomeruli. This inter- 

 pretation would not merely be in agreement with the described position 

 of the glomeruli with reference to the somites, but it would also accord 

 well with the figures Mihalkovics gives of the two sets of glomeruli. 

 Thus, in his representation of a transitional glomerulus (Taf. I. Fig. 17, 

 g. a).), there is little reason to regard the structure as in any way different 

 from a mesonephric Malpighian body. I may here further remark, that 

 nearly all other modern investigators agree in deriving a part, if not all, 

 of the mesonephros from a layer of cells which primitively bounded the 

 coelom, rather than from a strictly indifferent blastema. In this light, 

 the validity of the principal contrast Mihalkovics sought to establish be- 

 tween the pronephros and mesonephros becomes at least very uncertain. 

 The account of Janosik ('85) affords the best basis for the general 

 scheme I have proposed. The most anterior region, or pronephros, 

 develops somewhat later [!] than the first tubules of the mesonephros 

 (primary cords ?). The duct in the region of the pronephros is broken 

 up into fragments, which receive rudimentary peritoneal canals. Three 

 typical glomi are developed on the radix mesenterii. In the next follow- 

 ing region (intermediate), from two to five peritoneal canals communicate 

 with the Wolffian duct. Xear the nephrostomal ends of these canals 

 transitional glomeruli develop. Both the pronephros and the intermedi- 

 ate region rapidly atrophy. The remaining portion of the embryonic 

 excretory organ is the true mesonephros. The mesonephric tubules are 

 either developed as sepai-ate buds from the peritoneum, or are differen- 

 tiated from a blastema which is directly derived from the peritoneum. 

 Janosik was able to confirm Kenson's discovery of rudimentary pro- 

 nephric tubules in the Rabbit, but was unable to find in this form any 

 trace of external glomeruli. Later, however, he ('87, p. 582) described in 

 a young human embryo, 3 mm. in length, a peculiar projection into the 



