50 PROCEEDINGS OF THE ACADEMY OF [1887. 



produced by the nei^hroblasts ; these roAvs are designated as the 

 nephridial rows. In each somite a solid outgrowth from each ne- 

 phridial row takes place into the coelom and is ultimately conver- 

 ted into the nephridium of the corresi^onding side. Thus the 

 nephridia arise as metameric outgrowths from a solid cord of 

 cells that lies in the somatoj^leure ; and their mode of development 

 is therefore essentially similar to that of the segmental tubes of the 

 vertebrate head-kidney or pro-nephi^os. 



An examination of the origin of the nephroblast demonstrates the 

 fact that it is originally an ectoblastic cell, which extends to the 

 surface of the body and is only in rather late stages, to be distin- 

 guished from other ectoblastic cells by its greater size and by the 

 fact that it sinks below the surface. It always remains, however, 

 embedded in the ectoblast, and unquestionably is derived from that 

 layer. The nephridial rows and the nephridia to which they 

 give rise are therefore ectoblastic structures. 



This conclusion is believed by Prof Wilson to establish two 

 interesting homologies, namely: between the nephridial row of 

 I/umbricus and the Wolffian or segmental duct of the vertebrates, 

 and between the series of nephridia of annelides and the vertebrate 

 head-kidney or pro-nephros. It has very recently been shown that 

 in Ba/ja, Rana, Lacerta, guinea-pig and the rabbit, the segmental 

 duct is derived directly from the ectoblast by a mode of development 

 "essentially like that of the nephridial row of Lumhricus. Hatschek, 

 Eduard Meyer and Lang have already called attention to the close 

 reseml:)]ance between the Wolffian duct of vertebrates and the 

 longitudinal canal that unites the nephridia in the larval Polygordius 

 and in certain adult annelides. Prof. Wilson's observations, taken 

 in connection with those of Meyer supply the embryological proof 

 that the two structures are actually homologous, and that the 

 excretory systems of annelides and of vertebrates are constructed 

 upon fundamentally the same type and originate by similar modes 

 of development. Attention was called to the direct bearing of this 

 result on current theories relating to the origin of the Vertebrata. 



March 1. 

 The President, Dr Jos. Leidy, in the chair. 

 Twenty-six persons present. 



A paper entitled "The Summit Plates in Blastoids, Crinoids and 

 Cystids and their morphological Relations," By Charles Wachsmuth 

 and Frank Springer, was presented for publication. 

 The following was ordered to be printed : 



