MUSEUM OF COMPARATIVE ZOOLOGY. 279 



the pronephros, according to Sedgwick, lies between the 7th and 11th 

 protovertebrse and arises in the same region in which the duct first 

 appears. 



Lockwood ('87, pp. 657-GG3) describes three regions in the embryonic 

 excretory organ of the Rabbit. In the most anterior region (pronepliros), 

 the duct consists of isolated fragments, which are connected with the body 

 cavity by 2-3 nephrostomes. Then follows a region of typical nephrosto- 

 mal canals with glomeruli, and finally typical blind mesonephric tubules. 

 Possibly the last two regions belong to the mesonephros ; but in none 

 of the accounts of Mammalian development have I been able to recog- 

 nize with certainty the intermediate region. 



According to Renson ('83, p. 29), glomeruli develop in the Chick in 

 the region of the pronephros, which is otherwise described in agreement 

 with Sedgwick's account. The pronephric tubules atrophy with the 

 exception of their nephrostomes, and in the hollow of each funnel there 

 appears a glomerulus which soon comes to project freely into the body 

 cavity. In a region directly posterior to that in which the free glomeruli 

 occur, there are found the so-called mixed glomeruli, W'hich are situated 

 in the base of an infundibular depression, and are partially covered by 

 a fold of peritoneum. This, as well as the more anterior portion of the 

 system, Renson regards as belonging to the pronephros. He also de- 

 scribes in the Rabbit a series of peritoneal involutions in connection 

 with a discontinuous duct. In this region he likewise observed a vascu- 

 lar structure, which he regarded as a very rudimentary external glomer- 

 ulus. A similar observation has been recorded for human embryos by 

 Lockwood ('87, pp. 662, 663), and for Arvicola by Spoof ('83, p. 86, foot- 

 note). It is difiicult to arrive at a satisfactory estimate of Eenson's posi- 

 tion. There would be no difficulty in classing him with Sedgwick, were it 

 not for the circumstance that he describes for the pronephric region (6 or 

 7th to 11 or 12th somites) glomerular structures which, according to his 

 own comparison, develop in the same way as the transitional glomeruli 

 observed by Sedgwick in the "intermediate" region only (11th to 14th 

 somite). If, however, it should prove to be true that only the '^ mixed" 

 glomeruli develop in this way, the conflict would at once be removed, and 

 Renson's account would show the thi-ee primary regions in their typical 

 condition. 



According to Mihalkovics the most anterior two or three tubules (4-7 

 somites) in the Chick and Duck are derivatives of the communicating 

 canals, and gain a connection with the duct while yet opening into the 

 body cavity by a distinct ostium. The posterior canals, on the contrarv, 



