460 _ ADAM SEDGWICK, 
than a trace of the hinder part should appear simultaneously in 
embryonic development with the anterior part. If the rest of 
the mesonephros developed continuously with the duct and 
simultaneously with the pronephros, then, on the above hypo- 
thesis, we should not be able to distinguish a pronephros from 
the hinder part ; and it is opposed to all our ideas of economy to 
suppose that a rudiment of the mesonephros should appear at 
what phylogenetically would be the proper time, remaining over 
as a rudiment in the larva, z.¢. as a useless organ forming 
merely a burden until it was wanted. 
It seems to me that we can only expect, at the very utmost, 
to find a very small trace of the mesonephros in embryonic de- 
velopment at what phylogenetically we should consider, on the 
above hypothesis, to be the proper moment relative to the 
pronephros. 
I have been examining the development of the segmental 
duct in an Amphibian, the frog, to see if at the time of closure 
of the groove of the segmental duct any trace of a discontinuous 
closure such as we find in the head-kidney existed. If the 
pronephros is merely the anterior part of a segmental organ 
of which the mesonephros is the posterior part, and if 
phylogeny is in any way repeated in the development of the 
pronephros, we should expect to find that the discontinuous (seg- 
mented, see above) closure of the pronephros would be repeated 
behind, showing some traces at least of the openings of the 
segmental duct and of the specialised part of the body cavity 
which later forms the Wolffian tubule and contains the glome- 
rulus. So far it cannot be said that my search has been from 
my point of view successful. ‘To get any evidence of what I was 
searching for requires a very complete series of sections in a state 
of preservation favorable for observation. The difficulties pre- 
sented by the embryonic Amphibia in their early stages to such a 
successful result are very great. In the first place they are 
very brittle, and comparatively very few of the sections, even if 
thick, can be mounted uninjured. Of these, very few, indeed, 
can be obtained perfect, and those so obtained are apparently 
more difficult to see anything in than the thick ones. The cells 
are full of yolk granules which seem to escape and obliterate the 
outlines of the cells from the sight. 
While my results have not been such as to unable me to speak 
with any confidence either one way or the other, yet on the 
whole they have convinced me that a re-examination with a new 
method of the development of the segmental duct in Amphibia, 
&c., would repay the trouble. 
In the chick, on the other hand, the anterior part of the seg- 
mental duct, for the space of five segments, develops exactly in 
