KIDNEY IN RELATION TO WOLFFIAN BODY IN THE CHICK. 79 



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. 64) 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 apriori, 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 sim.ilar 

 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 

 from 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 



