640 EXISTENCE OF A HEAD-KIDNEY 



not enter the segmental duct at first by an opening specially 

 developed for them appears to us to follow from Dohrn's princi- 

 ple of the transmutation of function (Functionswec/isel). As a 

 consequence (by a process of natural selection) of the segmental 

 duct having both a generative and a urinary function, a further 

 differentiation took place, by which that duct became split into 

 two a ventral Miillerian duct and dorsal Wolffian duct. 



The Miillerian duct without doubt was continuous with the 

 head-kidney, and so with the abdominal opening or openings of 

 the head-kidney which served as generative pores. At first the 

 segmental duct was probably split longitudinally into two equal 

 portions, but the generative function of the Miillerian duct gra- 

 dually impressed itself more and more upon the embryonic 

 development, so that, in the course of time, the Miillerian duct 

 developed less and less at the expense of the Wolffian duct. 

 This process appears partly to have taken place in Elasmo- 

 branchii, and still more in Amphibia ; the Amphibia offering in 

 this respect a less primitive condition than Elasmobranchii ; 

 while in Aves it has been carried even further. The abdominal 

 opening no doubt also became specialised. At first it is quite 

 possible that more than one abdominal pore may have served for 

 the generative products ; one of which, no doubt, eventually came 

 to function alone. In Amphibia the specialisation of the open- 

 ing appears to have gone so far that it no longer has any 

 relation to the head-kidney, and even develops after the atrophy 

 of the head-kidney. In Elasmobranchii, on the other hand, the 

 functional opening appears at a period when we should expect 

 the head-kidney to develop. This state is very possibly the 

 result of a differentiation (along a different line to that in Am- 

 phibia) by which the head-kidney gradually ceased to become 

 developed, but by which the primitive opening (which in the 

 development of the head-kidney used to be divided into several 

 pores leading into the body-cavity) remained undivided and 

 served as the abdominal aperture of the Miillerian duct. Aves, 

 finally, appear to have become differentiated along a third line ; 

 since in their ancestors the anterior pore of the head-kidney 

 appears to have become specialised as the permanent opening 

 of the Miillerian duct. 



With reference to the posterior position of the head-kidney 



