144 THE URINOGENITAL ORGANS OF VERTEBRATES. 



In favour of this view are the following anatomical and em- 

 bryological facts, (i) It developes in nearly the same manner 

 as the other segmental tubes, viz. in Selachians as a solid 

 outgrowth from the intermediate cell- mass, which subsequently 

 becomes hollowed so as to open into the body-cavity : and in 

 Amphibians and Osseous and Cyclostome fishes as a direct 

 involution from the body-cavity. (2) In Amphibians, Cyclos- 

 tomes and Osseous fishes its upper end develops a glandular 

 portion, by becoming convoluted in a manner similar to the 

 other segmental tubes. This glandular portion is often called 

 either the head-kidney or the primitive kidney. It is only an 

 embryonic structure, but is important as demonstrating the true 

 nature of the primitive duct of the kidneys. 



We may suppose that some of the segmental tubes first 

 united, possibly in pairs, and that then by a continuation of this 

 process the whole of them coalesced into a common gland. 

 One external opening sufficed to carry off the entire secretion 

 of the gland, and the other openings therefore atrophied. 



This history is represented in the development of the dog- 

 fish in an abbreviated form, by the elongation of the first seg- 

 mental tube (segmental duct of the kidney) and its junction 

 with each of the posterior segmental tubes. Professor Semper 

 looks upon the primitive duct of the kidneys as a duct which 

 arose independently, and was not derived from metamorphosis 

 of the segmental organs. Against this view I would on the one 

 hand urge the consideration, that it is far easier to conceive of 

 the transformation by change of function (comp. Dohrn, Func- 

 tions^vechsel, Leipzig, 1875) of a segmental organ into a segmental 

 duct, than to understand the physiological cause which should 

 lead, in the presence of so many already formed ducts, to the 

 appearance of a totally new one. By its very nature a duct is a 

 structure which can hardly arise de novo. We must even sup- 

 pose that the segmental organs of Annelids were themselves 

 transformations of still simpler structures. On the other hand 

 I would point to the development in this very duct amongst 

 Amphibians and Osseous fishes of a glandular portion similar 

 to that of a segmental tube, as an a posteriori proof of its 

 being a metamorphosed segmental tube. The development in 

 insects of a longitudinal tracheal duct by the coalescence of a 



