HALL : MESOXEPHROS AND MULLERIAN DUCT IN AMPHIBIA. 119 



the primary ]\Ialpighian body at a point similar to that of the attachment 

 in the case of Rana. 



Brauer lays stress on the fact, likewise emphasized in my introduction, 

 that the inner tubule grows out from the lateral side of the blastula and 

 is comparable with the pronephric tubule, while the Malpighian body 

 (Bowman's capsule) is formed from the blastula by a direct transformation. 



The history of the secondary unit differs somewhat from that in Ambly- 

 stoma. After it is cut off from the primary, it lies close against the 

 Wolffian duct, posterior to the opening of the main tubule of the primary 

 unit. Where it touches the duct, a dark-staining evagination of that 

 organ grows out, pushing the blastula before it. A long, tubular 

 evagination of the Wolffian duct is thus formed into which the secondary 

 tubule later opens. Differentiation of the secondary blastula is very 

 similar to that of the primary. It develops in turn an evagination to 

 form the inner tubule, one to form the outer tubule and one which be- 

 comes the tertiary unit. The tertiary blastula develops an inner tubule 

 which meets and empties into a tubular outgrowth from the previous 

 evagination of the Wolffian duct. 



As many as eight units may be present in a single somite. Brauer is 

 not certain that any beyond the quaternary form outer tubules and 

 nephrostomes, but they may possibly connect with the body cavity in 

 the manner in which the most posterior secondary units occasionally do, 

 which is as follows. In Segments 90 to 100 the secondary outer tubule, 

 instead of joining the peritoneum directly, sometimes joins the outer 

 tubule of the primary unit, close to its nephrostome. 



In the posterior segments a much more common condition than the 

 one just described is the failure of both primary and secondary tubules 

 to connect with the peritoneum. Instead, the outer tubule of the 

 primary unit, avoiding the peritoneum, turns aside, meets and fuses 

 with the outer tubule of the secondary unit, thus putting the two Mal- 

 pighian bodies into connection with each other. 



The most important, and, to me, satisfactory result of Brauer's work, 

 the establi.shment of a close similarity between the mode of development 

 of tlie pronephric and mesouephric units, I may not discuss, as it would 

 carry us beyond the province of my paper*. There remains, however, the 

 develojyment of the Mullerian duct. The first process in the formation of 

 the Mullerian duct is a tliickening of the lateral wall of the anterior half 

 of the pronephros. On this thickened plate two longitudinal ridges arise. 

 The free edge of the upper one folds ventrad, that of the lower, dorsad. 

 On following the ridges caudad they are seen to form a tube by the 



