40 ADAM SEDGWICK. 



development of the organs not so required until after it is 

 hatched ; and in order that it may not be burdened by useless 

 organs, the cells from which the tubules after appear and which 

 should appear, if keeping the phylogenetic order, quite early in 

 embryonic life, in a way already indicated, are reduced so as 

 hitherto to have escaped observation. 



It is perfectly true that the pronephros does present peculia- 

 rities of structure not presented by the mesonephros, such as the 

 unsegmented nature of the glomerulus, and in the fact that the 

 tube connecting the cavity in which the glomerulus lies with 

 the segmental duct not being coiled. But in the fundamental 

 structure, i.e. in the possession of a glomerulus placed close to 

 the main vascular channel (aorta) , in the segmental arrangement 

 of the openings of the segmental duct into the cavity (anatomi- 

 cally corresponding in both cases) containing the glomerulus, 

 in the cavity containing the glomerulus being a specialised part 

 of the body cavity ; in all these points the pronephros and meso- 

 nephros resemble each other. 



Assuming for the moment the truth of this suggestion, we 

 find the pronephros to present that method of development 

 which a priori we are bound to assume would be if it were not 

 for disturbing causes, the development of the mesonephros, 

 because it represents the most probable method by which the 

 mesonephros and its duct can have arisen in phylogeny. 



The question now arises. What are the disturbing causes 

 which in Amphibia have so changed the phylogenetic develop- 

 ment ? The answer has already been given, but I will repeat it 

 here. It has been brought about by the action of natural selec- 

 tion on the innumerable larvae produced, so that only those ani- 

 mals reached the adult state which in their prelarval and larval 

 development conformed to the type of development we have 

 before us. 



Admitting the possibihty of both prelarval as well as larval 

 development varying at any particular stage, the tendency has 

 been to produce a dissimilarity in the early structure of the ex- 

 cretory organs of Elasmobranchii and Amphibia greater than 

 that which exists in the adult state, a result entirely in opposition 

 to what we should expect from the application of that principle 

 which has been laid down as regulating embryonic development, 

 viz. that embryos of different animals, starting as fairly similar, 

 become more and more dissimilar as their development proceeds. 



To get any actual proof from embryonic development in favour 

 of the above hypothesis must, from the nature of the case, be 

 very difficult. For the very reason of the existence of the pro- 

 nephros as an anterior part of the excretory system well marked 

 off from the posterior makes it improbable that anything more 



