78 ADAM SEDGWICK. 



3. Wolffian tubules do not appear in any part of the blas- 

 tema behind the thirtieth segment. Primary, secondary, and 

 tertiary, &c., tubules are developed in that part of it placed 

 in the thirtieth and anterior segments as far as the twenty- 

 first or twenty-second, and primary tubules in yet anterior 

 segments. 



4. The blastemain the thirty-first to thirty-fourth segments, 

 on the appearance of the ureter, moves dorsalwards from the 

 Wolffian duct, breaking away from the hindermost Wolffian 

 tubules and enters into close relation with the ureter. 



5. This part of the blastema — the kidney blastema — espe- 

 cially collects round swellings on the ureter, from which 

 kidney tubules grow out. 



6. These kidney tubules burrow into the kidney blastema, 

 their growing points being continuous with the cells of the 

 blastema. 



Five years ago Balfour^ and Semper^ independently put 

 forward the hypothesis that the kidney of the Amniota holds 

 the same relation to the embryonic Wolffian body as does the 

 adult kidney in Elasmobranchs. 



Balfour wrote then :^ " The last feature in the anatomy of 

 the Selachians which requires notice is the division of the 

 kidney into two portions, an anterior and posterior. The 

 anritomical similarity between this arrangement and that of 

 higher Vertebrates (birds, &c.) is very striking. The anterior 

 one precisely corresponds, anatomically, to the Wolffian body, 

 and the posterior to the true permanent kidney of higher 

 Vertebrates ; and when we find that in the Selachians the 

 duct for the anterior serves also for the semen, as does the 

 duct of higher Vertebrates, this similarity seems almost to 

 amount to identity." 



The development of the kidney of the bird has never 

 been fully worked out, so that this hypothesis, arrived at from 

 a consideration of the facts of comparative anatomy and Elas- 

 mobranch embryology, has hitherto not been tested by the 

 facts of Avian embryology. The observations described above 

 were undertaken with a view of testing this hypothesis, and, 

 in my opinion, it has fully stood the test. 



The development of the kidney in the chick points most 

 decidedly to the conclusion that it is merely the posterior 

 ])art of the Wolffian body — or, perhaps, it would be better to 

 say, of a primitive organ, the anterior part of which is now 



1 '' Urogenital Organs of Vertebrates," ' Journal of Anatomy and 

 Physiology,' vol. x. 

 ' Loo. eit. 

 5 P. 27. 



