WOLFFIAN DUCT AND BODY IN THE CHICK, 457 
ourselves obliged, for precisely similar reasons to those already 
given in the cage of the mesonephros, to suppose that that 
ontogeny is in this respect more primitive in which the duct 
arises as a continuous groove constricted off from the body 
cavity than that in which it arises as a solid knob (modified 
groove) for only a very small part of its course, and undergoing 
the major part of its early growth quite independently of sur- 
rounding structure. 
In Elasmobranchii that part which develops as a groove 
persists as a groove throughout life (abdominal opening of 
Miillerian duct). 
In Amphibia, &c., that part which develops as a groove 
becomes constricted off first in the middle, and then backwards 
and forwards, but in front it is constricted in a manner, 
according to Fiirbringer not understood, so as to leave the 
variable numbers of openings of the pronephros. 
However this may be, apparently the openings of the prone- 
phros develop as unclosed portions of the anterior end of the 
groove from which the duct arose, and they open into a space 
placed at the root of the mesentery close to the notochord and 
close to the point where in a previous stage the body cavity 
communicated with the muscle plates. 
In the Amphibian, and apparently in the Teleostean, there is 
no marked structure corresponding to the intermediate cell mass 
of Elasmobranchii. The muscle-plate cavity is, after its separa- 
tion from the general body cavity, only separated from the latter 
by a double layer of cells, forming its ventral wall and the wall 
of the body cavity ; 2. e. there is no portion of the body cavity 
at first continuous, but subsequently divided up by the coming 
together of its walls into a series of canals connecting the general 
body cavity with the muscle plates. 
Now the glomerulus of the pronephros develops in a part of 
the body cavity anatomically corresponding to the intermediate 
cell mass of Hlasmobranchii, only in Amphibia it does not, in 
this region, become divided up into chambers corresponding to 
the segments. 
With this part of the body cavity, from the somatic walls of 
which the original groove arose, the openings of the head-kidney 
communicate. The number of these openings corresponds with 
the number of segments occupied by the pronephros in all those 
animals in which they exceed one, except Myxine; but the 
development of the pronephros in Myxine is not at all known, 
and its adult structure is, on the whole, obscure. 
Turning again to Elasmobranchs, we find that the anterior 
knob of the segmental duct arises from the intermediate cell 
mass, 7. é. from a part of the body cavity corresponding serially 
