THE URINOGENITAL ORGANS OF VERTEBRATES. 167 



The only further point which requires discussion is the em- 

 bryonic layer from which these organs are derived. 



I have shewn beyond a doubt (loc. cit^) that in Selachians 

 these organs are formed from the mesoblast. The unanimous 

 testimony of all the recent investigators of Amphibians leads to 

 the same conclusion. In birds, on the other hand, various in- 

 vestigators have attempted to prove that these organs are 

 derived from the epiblast. The proof they give is the fol- 

 lowing : the epiblast and mesoblast appear fused in the region 

 of the axis cord. From this some investigators have been led 

 to the conclusion that the whole of the mesoblast is derived 

 from the upper of the two primitive embryonic layers. To 

 these it may be replied that, even granting their view to be 

 correct, it is no proof of the derivation of the urinogenital 

 organs from the epiblast, since it is not till the complete for- 

 mation of the three layers that any one of them can be said to 

 exist. Others look upon the fusion of the two layers as a proof 

 of the passage of cells from the epiblast into the mesoblast. 

 An assumption in itself, which however is followed by the further 

 assumption that it is from these epiblast cells that the urino- 

 genital system is derived ! Whatever may have been the primi- 

 tive origin of the system, its mesoblastic origin in vertebrates 

 cannot in my opinion be denied. 



Kowalewsky (Embryo. Stud, an Vermeil u. ArtJiropoda, Mem. 

 Akad. St Petersbourg, 1871) finds that the segmental tubes of 

 Annelids develope from the mesoblast. We must therefore look 

 upon the mesoblastic origin of the excretory system as having 

 an antiquity greater even than that of vertebrates. 



