306 BULLETIN OF THE 



is usually regarded adequate to show the existence in the pronephric 

 region of metameric diverticula proceeding from the body cavity towards 

 the overlying protovertebree. These diverticula are the communicating 

 canals, and it is undoubtedly true that from similar canals in the pos- 

 terior region mesonephric tubules are actually developed ; but, to my 

 mind, the occurrence of these diverticula in the pronephric region cannot 

 be brought forward as evidence of the existence of two sets of nephridial 

 tubules in these somites, until it can be shown that these remnants of 

 the canal-like commiuiication between protovertebrae and lateral plates 

 exhibit some indication of the characteristic nephridial differentiation, 

 i. e. grow outward and join the duct. This, I believe, has never been 

 demonstrated. The existence of such a growth has, however, been as- 

 serted by several observers; but it seems to me compatible with the 

 view I have expressed of the relations between pronephros and mesoneph- 

 ros. Since the time of the investigations of Balfour and of Semper on 

 Selachians, it has been a familiar fact, that, although at first only one 

 mesonephric tubule occurs in each somite, the further complication of 

 the gland is largely produced by the formation of new tubules which 

 proceed from the region of the primary Malpighian capsule. If the 

 development of more than one tubule in a somite became normal in the 

 ancestors of the Craniotes before the separation of pronephros and meso- 

 nephros took place, the development of such secondary tubules in the 

 pronephric region would at once be intelligible. 



A more fundamental objection is contained in an ignored observation 

 of Gasser ('82, p. 96) on Alytes, according to which a typical glomus is 

 developed in the body cavity of the mesonephric region, in addition to the 

 universally present glomeruli. Gasser's account is contained in a rather 

 short note unaccompanied by figures ; it has not been confirmed by any 

 subsequent observer ; nor have I been able to find such a structure in 

 either Eana or Amblystoma. I am therefore inclined to the opinion 

 that Gasser may have mistaken for the glomus either the germinal ridge 

 or the fat-body, both of which are developed in this region, although this 

 explanation would contradict the statement of Gasser that the mesoneph- 

 ric glomus is a transitory organ. Be that as it may, I cannot without 

 further evidence accept his account as final. 



Semon ('90) has recently asserted that the pronephros and mesoneph- 

 ros are built upon the same structural type. He was led to this con- 

 clusion by a study of the excretory system in Ichthyophis. I have 

 already alluded to the condition of the pronephros in this form. It is 

 characterized by tiie possession of a completely closed pronephric cham- 



