MUSEUM OF COMPARATIVE ZOOLOGY. 293 



In Reptiles, a number of writers have asserted that the Wolffian duct 

 arises from the ectoderm. According to Perenyi ('87, '88, '89), irregu- 

 lar groups of cells are at an early stage budded off from the ectoderm 

 covering the middle plate, and on the first formation of the segmental 

 vesicles they form the cord of cells which has been recognized by prior 

 writers as the fundament of the duct. In my opinion, no conclusive 

 evidence is adduced to prove that the cells figured in the latter position 

 ('89, Fig. 5, ceW.) are descendants of those which at an early stage 

 form part of the ectodermal thickening. Mitsukuri ('88) and Orr ('87) 

 have published short notes claiming an ectodermal origin for the duct ; 

 and, finally, Ost roundoff ('88 a , '88 b ) asserts that it is derived from the 

 ectoderm in Phrynocephalus. It seems to me, however, that Ostroumoff's 

 observations are incomplete at a critical point, and that no satisfac- 

 tory evidence is brought forward to show that the ectodermal thick- 

 enings which he describes and figures ('88 b , Tab. III. Fig. 5G) with all 

 desirable clearness, are unquestionably the fundament of the Wolffian duct. 

 They may be merely chance thickenings over the intersegmental depres- 

 sions in the underlying mesoderm. On the other hand, Mihalkovics 

 ('85), Strahl ('8G), and Hoffmann ('89) have all sought in vain to find 

 satisfactory evidence of a participation of the ectoderm in the formation 

 of the Wolffian duct. 



With all the preceding classes of Vertebrates, I am of opinion that 

 the weight of evidence is at present in favor of the view that the excre- 

 tory system is wholly derived from the mesoderm. For the remaining 

 groups, Mammals and Selachians, however, no such claim can be sus- 

 tained. The researches of Graf Spee on Cavia showed conclusively 

 that a cord of cells representing the fundament of the Wolffian duct is 

 continuous posteriorly with a ridge of tissue which is still in intimate 

 union with the superficial ectoderm, and that, in the further develop- 

 ment, a continuous cord of cells separates off from this ridge by the pro- 

 gressive formation of a split between the deep portion of the ridge and 

 the superficial ectoderm. At first, a distinct membrana prima is reflected 

 from the unmodified ectoderm over the ridge, and the partially separated 

 fundament of the duct may still be in connection with the superficial 

 ectoderm by means of such a membrane. Tins latter feature is also 

 dwelt upon bv Flemming ('86), who furthermore emphasizes the cir- 

 cumstance that in the ridge which forms the first rudiment of the 

 Wolffian duct mitoses are especially abundant, and that the nuclear 

 spindles are frequently perpendicular to the surface, i. e. are so situated 

 that the ensuing cell divisions would tend to thicken the layer. The 



