282 BULLETIN OF THE 



In opposition to this view, I would maintain : (1) that the first trace 

 of the excretory system consists of a solid proliferation of somatopleure, 

 the pronephric thickening ; (2) that the lumen of the system arises 

 secondarily ; and (3) that the pronephric tubules do not appear in con- 

 sequence of the local fusion of the walls of a widely open pouch, but 

 that they are differentiated at an early stage from the hitherto indifferent 

 pronephric thickening. 1 



The development of the pronephros and duct from a solid mass of 

 mesoderm was a common feature in the accounts of those who wrote 

 prior to Willi. Midler and Goette, but since then this mode of origin, 

 though repeatedly maintained by single observers, has failed to gain 

 general acceptance. Clarke ('81) described a solid pronephric thickening, 

 and asserted that the lumen arose secondarily in this mass; the details 

 of the process are, however, not accurately given. Duval ('82) also 

 described the pronephros as first appearing in the form of a solid thick- 

 ening. He however states that it later acquires a slitdike opening into 

 the body cavity, and that by the imperfect closure of this opening the 

 successive nephrostomies are formed, as described by Goette and Fiir- 

 bringer. This latter statement I am unable to confirm. Gasser's ('82, 

 pp. 89-97) short note gives, on the other hand, an account of the early 

 development in Alytes, which is in substantial agreement with my own 

 observations. His account of the first differentiation of the nephrostomal 

 canals is not very full, but it is not improbable that he conceived it to 

 take place in a manner altogether similar to that which 1 have described. 

 His statements seem to me in general correct, 2 but incomplete. 



Janosik ('85, p. 19) states, on the basis of personal observations, that 

 the first trace of the segmental duct in Bufo and Triton is a solid mass 

 of cells, which he is, however, incliued to regard as a disguised fold of 

 somatopleure. 



According to a recent account by Kellogg ('90), a lumen does not 

 appear anywhere in the organ (except in the region of the nephrostomes) 

 until it has been separated from the peritoneum. Finally, Mollier ('90, 

 Ruckert and Mollier, '89) has published an account of the early develop- 

 ment, which is for the most part in close accord with the results of my 

 own studies. Since these results were gained entirely independently of 



1 The large cavity which the pronephric pouch presents in Stage IV. of Rana 

 and Bufo is a secondary condition produced by the expansion of the lumens of the 

 several diverticula. 



- 1 must here except his statement that the second tubule is differentiated before 

 the rest; this I believe to be an error. 



