Sidney in relation to wolffian body in the chick. 16 S 
seen as the Wolffian body — the whole of this primitive organ 
in Elasmobranch embryos being termed Wolffian body. 
The most important fact in favour of Balfour’s hypothesis 
is the primitive continuity in early stages in the bird of the 
cells from which both Wolffian body and kidney arise. 
The differences in later development cannot be looked 
upon as a serious difficulty when we remember the immense 
differences which many undoubtedly homologous organs 
show in their embryonic development in various animals. 
It has been stated (see above, p. 148) by many students 
of Avian embryology that the kidney tubules develop as 
outgrowths from the ureter, and that the cells of the kidney 
blastema merely give rise to the vascular elements of the 
glomerulus. This view, whether considered a priori, or with 
reference to the facts of development, cannot for a moment 
be maintained. 
If Balfour’s hypothesis as to the relation of the kid- 
ney to the Wolffian body be accepted, and I do not see 
how it can be rejected, assuming the truth of the facts 
of development recorded in this account, it would re- 
quire very strong proof indeed to establish the fact that 
the cells of the kidney blastema give rise merely to the 
vascular elements of the glomerulus, and take no part in 
the formation of the secretory epithelium of the kidney 
tubules, such as is taken in the formation of tubules of a 
very similar organ by cells developed in a precisely similar 
way. Such proof is not forthcoming, and would be very 
hard to give. 
Considering the very late development of the posterior 
part of the Wolffian body (kidney) in the chick with refer- 
ence to that of the anterior part, it surely cannot be a matter 
of surprise if the development has been modified, the 
walls of the tubule arising from the cells of the blastema ; 
the lumen, however, not as in the anterior part, first appear- 
ing as an independent cavity, which opens later into the 
duct, but being from the first continuous with the lumen of 
the ureter. 
Fiirbringer’s suggestion, that the Amniote kidney is de- 
rived from dorsal tubules of the Wolffian body, is based 
mainly on the fact that it lies dorsal to the Wolffian body, 
and an observation of Braun’s for Lizards. Braun has 
stated for these animals that the kidney blastema develops 
Irom irregular ingrowths of the peritoneal epithelium, at a 
period when the secondary dorsal tubules of the Wolffian 
body are developing. 
With regard to the first point it is to be noted that the 
