MUSEUM OF COMPARATIVE ZOOLOGY. 275 



Wijhe, the mesoderm which produces the mesonephric tubules in Selachii 

 belongs to a region dorsal to that which gave rise to the pronephros 

 (see the diagrams appended to van Wijhe, '89, Taf. XXXIL). 



In view of the difficulties to which I have alluded, it seems to me that 

 Hoftmann's position cannot be regarded as satisfactory. Furthermore, if 

 Hoftmana's observations^ on the origin of the posterior mesonephric 

 tubules be accurate, the contrast which Mihalkovics endeavored to es- 

 tablish between the anterior and posterior tubules does not exist. If, 

 finally, these anterior three or four pairs of tubules develop in their 

 course typical Malpighian capsules remote from the peritoneum, — Mihal- 

 kovics is not clear on this point, — I can see no reason for regarding them 

 as pronephric. I am thei-efore of opinion that there is at present no 

 evidence which j)i^oves a pronephros to exist either in Lacertilia or in 

 Ophidia. 



It remains for me to consider two recent papers by Wiedersheim 

 ('90% '90''), which describe a very interesting condition of the excretory 

 system in Crocodilia and Chelonia. The anterior portion of the em- 

 bryonic excretory organs in these groups consists of a number of 

 tubules which take their origin in ciliated nephrostomes, and, after un- 

 dergoing contortion, join a longitudinal canal continuous with the seg- 

 mental duct. From the root of the mesentery a large glomus protrudes 

 into the body cavity. It lies in a distinct fold of the peritoneum, and 

 consists of a mass of highly vascular tissue receiving distinct vessels 

 from the aorta. It extends continuously opposite a number of nephro- 

 stomes, and is evidently equivalent to the Amphibian glomus. In some- 

 what more posterior regions the conditions are essentially the same ; 

 but the nephrostomes and the glomus having approached each other, 

 they are cut off from the main portion of the body cavity by a longitu- 

 dinal fold of peritoneum. In this manner, there is formed a pronephric 

 chamber comparable to that of Amphibia. In yet more posterior regions, 

 the pronephric chamber with its contained glomus breaks up into a series 

 of capsules containing glomeruli, each of which then appears to form the 

 blind termination of a tubule. This is the region of the mesonephros with 

 typical Malpighian capsules. In the subsequent development of the em- 

 bryo, the anterior portion of this excretory system early ati'ophies, and 

 the hinder part alone constitutes the well known Wolffian body, or 

 rnesonephros. In my opinion, the account given by Wiedersheim affords 

 a satisfactory basis for the view that the most anterior portion of this 

 excretory system is truly pronephric. It seems, however, quite impos- 



1 Similar observations are recorded by Orr ('87, pp. 325-327). 



