MUSEUM OF COMPARATIVE ZOOLOGY. 241 



The glomus is somewhat larger and more compact than in the preced- 

 ing stage, and for that reason its structure is more obscure ; but I have 



© © ' 



seen nothing which would lead me to believe that it differs materially 

 from the condition exhibited by the younger glomi of Stage V. The 

 organ is bounded by a definite peritoneal layer and contains blood cells 

 together with embryonic connective-tissue stroma. The blood cells are 

 usually contained in definite channels, and, being closely packed together, 

 frequently appear in cross sections to be disposed with considerable reg- 

 ularity around a central point. This arrangement is naturally suggest- 

 ive of a tubular or a rod-like structure ; but the histological characters 

 of the cells and the conditions exhibited by adjacent sections show that 

 this impression is illusory. In short, I have been unable to find within 

 the glomus any traces of the rods and thick-walled tubes which have 

 been described by Hoffmann ('86, p. 591). 



No closed pronephric chamber exists at this stage. In the most 

 anterior sections in which the pronephric tubules appear, a blind anterior 

 diverticulum of the body cavity is to be seen ; but this unites with the 

 general body cavity surrounding the intestine even before the niveau 

 of the first nephrostome is passed. Throughout the remainder of the 

 pronephric region the lung bud (Plate IV. Fig. 28, fuel, pul.) forms a 

 ridge on the splanchnic side of the coelom. This ridge partially separates 

 the pronephric chamber from the general body cavity ; and in the region 

 of the third nephrostome a still more perfect closure is effected on the 

 right side of the body by means of the approximation of a portion of the 

 midgut to the peritoneum covering the pronephros. 



Stage VII. 



The age of the larvse of this stage, reckoned from the time of fertiliza- 

 tion, is about forty-seven days. A large gap therefore intervenes between 

 Stages VI. and VII., and the older larva are studied merely for the pur- 

 pose of observing the process of degeneration in the pronephros. In the 

 larvse of Stage VII. the mesonephros has already attained a degree of 

 complication comparable to that gained by the pronephros at Stage VI., 

 i. e. the same average number of tubes appear in cross sections through 

 the two glands. The mass of contorted tubules in the case of the meso- 

 nephros, however, is formed wholly by the transverse tubules, while the 



communication with the cardinal vein. In a larva of intermediate age, the sinus 

 communicates with the cardinal vein by means of a very narrow canal. Respect- 

 ins the fate and the significance of this singular structure. I have no suggestions to 

 offer. 



VOL. XXI. — NO. 5. 16 



