WOLFFIAN DUCT AND BODY IN THE CHICK, 459 
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, 7.¢. 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 @ 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 larve 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 possibility of both prelarval as well as larvul 
development varying at any particular stage, the tendency has 
been to produce a dissimilarity in the early structure of the ex- 
cretory organs of Hlasmobranchii and Amphibia greater than 
that which exists in the adult state, a result entirely m 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 
VOL. XXI,—NEW SER. H A 
