WOLFFIAN DUCT AND BODY IN THE CHICK. 49 



meiit was consequently modified. This part is known to us as 

 the mesonephros. 



The same hypothesis was applied to account for the retarda- 

 tion and modification of the development of the metanephros 

 with regard to the mesonephros in the Amniota. 



The main facts in favour of the hypothesis are — 



1. The development of the segmental tubes in Elasmobranchii 

 and of the pronephros and segmental duCt of the Ichthyopsida as 

 parts of the body cavity. 



2. The obvious modification in development of the meso- 

 nephros, accompanying also the presence of a pronephros in 

 most of the Ichthyopsida. 



3. The resemblance in structure between the pronephros and 

 mesonephros, particular stress being laid on the fact that the 

 glomerulus in both glands is developed in anatomically/ corre- 

 sponding, i.e. homologous, parts of the body cavity 



I may point out before leaving the subject that other views con- 

 cerning thenatureof the pronephros have been expressed by Gegen- 

 baur, Fiirbringer/ and Balfour.^ The two former authors look 

 upon the pronephros as having an antiquity greater than that of 

 Vertebrates, greater even than that of the segmented ancestors 

 of Vertebrates. They regard it as being descended from the 

 primitive excretory system possessed by the unsegmented 

 ancestor, which has been retained in such forms as Turbellaria 

 and Eotifera, the segmented posterior part having been added 

 when the segmented state was reached. 



Milllerian Duct. 



Balfour's views as to the phylogeny of the Miillerian duct 

 and its homology throughout the Vertebrata are well known. 

 He supposes it is one or, in the chick, more of the head-kidney 

 openings which have become modified for generative purposes. 



I still adhere to the view expressed in the paper on the 

 "Rudimentary Head-Kidney of the Chick'' as to the meaning 

 of the peculiar structures at the anterior end of the Milllerian 

 duct, and I think that there are grounds, which it is not necessary 

 to enter into here, for supposing that the abdominal opening or 

 openings of the Miillerian duct have been derived from the 

 anterior part of the excretory system after its modification to 

 form the pronephros. But I quite admit that a fuller know- 

 ledge of the early development of the Elasmobranch segmental 

 duct may necessitate an alteration in this view. 



* Loc. cit. 



^ Balfour looks upon it as the most primitive part of the excretory 

 system wliich has been retained by the larva, as so many ancestral organs 

 are, long after they have been lost by the adult. 'Comparative Embryo- 

 logy,' vol. ii. 



5 



