EXISTENCE OF HEAD-KIDNEY IN THE EMBHYO CHICK. 19 



of the Miillerian duct is formed, we would suggest the following 

 considerations : 



The history of the development of the excretory system teaches 

 us that primitively the segmental duct must have served as 

 efferent duct both for the generative products and kidney secre- 

 tion (just as the Wolffian duct still does for the testicular pro- 

 ducts and secretion of the Wolffian body in Elasmobranchii and 

 Amphibia) ; and further, that at first the generative products 

 entered the segmental duct from the abdominal cavity by one 

 or more of the abdominal openings of the kidney (almost cer- 

 tainly of the head-kidney). That the generative products did 

 not enter the segmental duct at first by an opening specially 

 developed for them appears to us to follow from Dohrn^s 

 principle of the transmutation of function (Functions wechsel). 

 As a consequence (by a process of natural selection) of the seg- 

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

 further difi'erentiation 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 deve- 

 lopment, 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 Elasmobranchii, 

 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 opening 

 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; 



