HALL : MESONEPHROS AND MULLEKL\N DUCT IN AMPHIBIA. 119 



the primary Malpighian body at a point similar to that of the attachmeut 

 in the case of Rana. 



Brauer lays vstress 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 tliat 

 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 tlie 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 

 uephrostomes, 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 moi'S common condition than the 

 one just described is the failure of both primary anci 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 establishment of a close similarity between the mode of development 

 of the pronephric and mesonephric units, I may not discuss, as it would 

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

 flerelojmenf of the Miillerian dud. The first process in the formation of 

 the Miillerian duct is a thickening 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 



