270 BULLETIN OF THE 



ectoderm and probably receives a contribution of cells from tbat layer. 

 The duct grows backwards as far as the cloaca at the expense of the 

 ectoderm. Having reached this stage of development, the pronephros 

 rapidly degenerates. This process takes place in a slightly different way 

 in the anterior and posterior regions. A variable number of the most 

 anterior evaginations flatten out into a simple longitudinal groove of per- 

 itoneum, the ostium abdominale ; the remaining ones become closed and 

 detached from the peritoneum ; thus there remains a longitudinal canal 

 communicating with the body cavity by the slit-like ostium. In inter- 

 preting the structure as a rudimentary pronephros, it is important to 

 note the discovery by Ruckert (pp. 239-242) of a structure which he re- 

 gards as a pronephric glomerulus, or glomus. This structure is developed 

 in connection with segmental blood-vessels which pass from the aorta to 

 the right subintestinal vein, and which have been described by Paul 

 Mayer ('87, p. 343). In Torpedo the vessels are present on the right side 

 in the same number as the segments of the pronephros, and as they pass 

 ventrally between the entoderm and the splanchnopleure it is to be noticed, 

 in regard to the middle vessels at least, that they send out buds, which 

 form projections from the median peritoneal wall opposite the pronephric 

 tubules. 



It will be at once seen that the development of the pronephros as de- 

 scribed by Ruckert is in striking agreement with the account I have 

 given of the early stages in the development of the Amphibian pro- 

 nephros, and I have no hesitation in homologizing the two organs. The 

 earliest stage which has been observed in both groups is that which I 

 have termed the pronephric thickening. This is followed in both by the 

 stage of canalization ; but the Selachian pronephros never goes beyond 

 an early condition of the pronephric pouch, in which, however, the homo- 

 logues of the nephrostomal tubules and the collecting trunk appear. 

 The points of difference between the account I have given and that 

 given by Ruckert for corresponding stages of the Selachian pronephros 

 seem to me, with a single exception, to be either unreal or insignificant. 

 The exception to which I refer pertains to the participation of the ecto- 

 derm in the formation of the pronephric thickening. This condition I 

 am confident does not occur in Amphibia. Moreover, the evidence upon 

 which Ruckert bases his statement seems to me far from conclusive, nor 

 has his observation been . confirmed by any subsequent investigator. 

 Ruckert described the pronephric thickening as a product of the proto- 

 vertebra?. I cannot admit that this is true for Amphibia ; but I believe 

 that our differences of opinion are really due to the fact that we use dif- 



