634 EXISTENCE OF A HEAD-KIDNEY 



The abdominal aperture of the segmental duct in Elasmo- 

 branchii, in that it becomes the permanent abdominal opening 

 of the oviduct, corresponds physiologically rather with the 

 abdominal opening of the Miillerian duct than with that of the 

 segmental duct of Amphibia, which, after becoming divided up 

 to form the pores of the head-kidney, undergoes atrophy. Mor- 

 phologically, however, it appears to correspond with the opening 

 of the segmental duct in Amphibia. We shall allude to this 

 point more than once again, and give our grounds for the above 

 view on p. 640. 



The development of the segmental duct in Elasmobranchii 

 as a solid rod is, we hope to shew, of special importance for the 

 elucidation of the excretory system of Aves. 



The development of these parts of Petromyzon is not fully 

 known, but from W. Miiller's account (Jcnaische Zeitschrift, 

 1875) it would seem that an anterior invagination of the peri- 

 toneal epithelium is continued backwards as a duct (segmental 

 duct), and that the anterior opening subsequently becomes 

 divided up into the various apertures of the head-kidney. If 

 this account is correct, Petromyzon presents a type intermediate 

 between Amphibia and Elasmobranchii. In certain types, viz. 

 Marsipobranchii and Teleostei, the segmental duct becomes the 

 duct for the posterior kidney (segmental tubes), but otherwise 

 undergoes no further differentiation. In the majority of types, 



growth, which easily leads to the supposition of a structure formed anteriorly forcing 

 its way through the tissues behind. This is, however, not the case, since, to my 

 knowledge, no author has ever detected a sharp boundary between the growing point 

 of the segmental duct (or Miillerian duct) and the surrounding tissues." He goes on 

 to say that " the growth in these cases really takes place by a differentiation of tissue 

 along a line in the region of the peritoneal cavity." Although I fully admit that it 

 would be far easier to homologise the development of the segmental duct in Amphibia 

 and Elasmobranchii according to this view, I must nevertheless vindicate the accuracy 

 of my original account. I have looked over my specimens again, since the appear- 

 ance of Dr Furbringer's paper, and can find no evidence of the end of the duct 

 becoming continuous with the adjoining mesoblastic tissues. In the section, before 

 its disappearance, the segmental duct may, so far as I can make out, be seen as a 

 very small but distinct rod, which is much more closely connected with the epiblast 

 than with any other layer. From Gasser's observations on the Wolffian duct in the 

 bird, I am led to conclude that it behaves in the same way as the segmental duct in 

 the Elasmobranchii. I will not deny that it is possible that the growth of the duct 

 takes place by wandering cells, but on this point I have no evidence, and must there- 

 fore leave the question an open one. F. M. B. 



