WOLFFIAN DUCT AND BODY IN THE CHICK, 463 
planation, founded on facts of development, can be given of it. 
I will suggest a possible, but entirely rough and hypothetical, 
solution on the lines so far followed. 
Before the Elasmobranchii produced eggs with the large food 
yolk they at present possess, they may have undergone a large 
part of their development in the surrounding medium as free 
larvee. These larve must have left the egg at a time when 
the cavities of the muscle plates were still open to the body 
cavity, and when the segmental duct had only just commenced 
to be formed in front, and before the development of the vascular 
system, and therefore before the glomerulus, the functions of 
which were probably carried on by the walls of the body cavity. 
The segmental duct was quickly developed from a groove into a 
duct, the larve thus precociously developing a recently acquired 
adult structure. With this constitution the larva of the ances- 
tral Elasmobranch quickly developed the rest of its excretory 
system. In consequence of the larva having been hatched at a 
very primitive stage, before the muscle plates were separated 
from the body cavity, certain primitive characters in the develop- 
ment of the segmental tubes were retained. These characters 
have been more or less transmitted to the present day, this 
having been rendered possible by the acquisition of food yolk 
and abolition of the larval state. 
However this may be, and it is useless now to make hypo- 
theses of this kind, we can only wait till a more close study of 
Klasmobranch development has been made to see if any traces 
can be found of the disturbing cause which has produced the 
modification in the development of the excretory system assumed 
on the above hypothesis, and very possibly in the search along 
the lines which this hypothesis indicates quite a different view as 
to the phylogeny of the vertebrate excretory system may pre- 
sent itself. 
Before concluding I will briefly state what I think to have 
been the structure of the primitive excretory system in the 
ancestral Vertebrate. 
There was a duct occupying the position of the segmental 
duct, i.e. at the dorsal outer angle of the body cavity, at the 
point where the latter becomes separated from the cavities of 
the muscle plates. This duct opened in each segment into the 
dorsal part of the body cavity. On the inner wall of the latter 
projected on each side a vascular ridge formed by the aorta. 
Behind, the segmental duct opened into the cloaca. 
As differentiation proceeded the vascular aortic ridge became 
more especially developed opposite each opening of the segmental 
duct, and parts of each of these enlargements became succes- 
sively enclosed in a special part of the body cavity, giving rise 
