602 GENERAL CONCLUSIONS. 



Physiological considerations appear to shew that of these three 

 methods of development the first is the most primitive. The deve- 

 lopment of the tubes as solid cords can hardly be primary. 



A question which has to be answered in reference to the segmental 

 tubes is that of the homology of the secondarily developed peritoneal open- 

 ings of Amphibia, with the primary openings of the Elasmobranchii. It 

 is on the one hand difficult to understand why, if the openings are homologous 

 in the two types, the original peritoneal attachment should be obliterated 

 in Amphibia, only to be shortly afterwards I'eacquired. On the other 

 hand it is still more difficult to understand what physiological gain there 

 could be, on the assumption of the non-homology of the openings, in the 

 replacement of the primary opening by a secondary opening exactly similar 

 to it. Considering the great variations in develoi)ment which occur in 

 undoubtedly homologous parts I incline to the view that the openings in 

 the two types are homologous. 



In the majority of the lower Vertebrata the mesonephric tubes have 

 at first a segmental arrangement, and this is no doubt the primitive 

 .condition. The coexistence of two, three, or more of them in a 

 I single segment in Amphibia, Aves and Mammalia has recently been 

 ssliewn, by an interesting discovery of Eisig, to have a parallel 

 (amongst Choetopods, in the coexistence of several segmental organs 

 (in a single segment in some of the Capitellidse. 



In connection with the segmental features of the mesonephros it is 

 perhaps worth recalling the fact that in Elasmobranchii as well as 

 other types there are traces of segmental tubes in some of the 

 postanal segments. In the case of all the segmental tubes a Mal- 

 pighian body becomes established close to the extremity of the tube 

 adjoining the peritoneal opening, or in an homologous position in 

 tubes without such an opening. The opposite extremity of the tube 

 always becomes attached to the segmental duct. 



In many of the segments of the mesonephros, especially in the 

 hinder ones, secondary and tertiary tubes become developed in 

 certain types, which join the collecting canals of the primary tubes, 

 and are provided, like the primary tubes, with Malpighian bodies at 

 their blind extremities. 



There can it appears to me be little or no doubt that the 

 secondary tubes in the different types are homodynamous if not 

 homologous. Under these circumstances it is surprising to find in 

 what different ways they take their origin. In Elasmobranchii a bud 

 sprouts out from the Malpighian body of one segment, and joins the 

 collecting tube of the preceding segment, and subsequently, becoming 

 detached from the Malpighian body from which it sprouted, forms 

 a fresh secondary Malpighian body at its blind extremity. Thus the 

 secondary tubes of one segment are formed as buds from the segment 

 behind. In Amphibia (Salamandra) and Aves the secondary tubes 

 develop independently in the mesoblast. These great differences in 

 development are important in reference to the homology of the meta- 

 nephros or permanent kidney, which is discussed below. 



