WOLFFIAN DUCT AND BUDY IN THE CHICK, 467 
phros, then, is to be explained on the supposition that the larva 
of the chick’s ancestor used it for a considerable period of its 
early life as an excretory organ, so that it may be said that the 
pronephros holds the same general relation to the mesonephros 
in the Ichthyopsida as does the mesonephros to the metane- 
phros in the Amniota. 
I do not mean to affirm that the above explanation of the 
lateness of the development of the metanephros is absolutely 
valid, for I think that a careful consideration of the develop- 
ment of the hind part of the mesonephros in Amphibia and Elas- 
mobranchii might necessitate a slightly different explanation. 
But an explanation of that kind must be sought to explain the 
remarkably late development in the chick of an organ which 
phylogenetically must be assumed to have had an origin simul- 
taneous with that of the mesonephros. 
With regard to the relation which the testes enters into-with 
the mesonephros, it is interesting to notice the modified develop- 
ment which always characterises this connection. 
Here it can be definitely affirmed that the lateness and conse- 
quent modification of the process is due to the fact that the 
apparatus has not been required in the larve of the Ichthyopsida 
and of the Amniote ancestors, and consequently has been put off 
and modified in development. The explanation is exactly similar to 
that given for the modification in development of the Amphibian 
mesonephros, except that here we are supposed to be able to 
assert with greater reason that the putting off and consequent 
modification is due to the fact that the connection between the 
testes and mesonephros was not wanted sooner, and so was not 
developed. . 
Summary of the Hypothesis and main Arguments used. 
The whole of the Vertebrate excretory system, including pro- 
nephros, mesonephros, and metanephros, are derived from a 
primitive organ possessed by the ancestral Vertebrate. This 
organ had a segmental character, and consisted of a duct, the 
segmental duct opening in every segment into the body cavity, 
close to a continuous structure, known now as the glomerulus, 
which was placed close to the main vascular channels and acted 
as an excretory organ. 
The anterior end of this organ was used by the larva, and 
developing more or less with regard to other structures at the 
normal time, retained many primitive features of development 
originally characterising the whole organ, and is known to us as 
the pronephros. The posterior part of the organ had its develop- 
ment delayed with regard to other structures, particularly those 
in connection with which it primitively developed ; the develop- 
