l4 BALFOUR AND SEDGWICK. 



towards the epiblast^ and the column connected with it lies 

 between the mesoblast and epiblast. The knob and column do 

 not long remain solid, but the former acquires an opening into 

 the body-cavity continuous with a lumen, which makes its appear- 

 ance in the latter/^i 



The difference in the development of the segmental duct in the 

 two types (Amphibia and Elasmobranchii) is very important. In 

 the one case a continuous groove of the peritoneal epithelium 

 becomes constricted into a canal, in the other a solid knob of 

 cells is continued into a rod, at first solid, which grows backwards 

 without any apparent relation to the peritoneal epithelium. 



The abdominalaperture of the segmental duct in Elasmobranchii, 

 in that it becomes the permanent abdominal opening of the ovi- 

 duct, corresponds physiologically rather with the abdominal open- 

 ing of the Mllllerian 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. Morphologically, 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. 19. 



The development of the segmental duct in Elasmobranchii as a 

 solid rod is, we hope to show, of special importance for the 

 elucidation of the excretory system of Aves. 



The development of these parts Petromyzon is not fully 

 known, but from W. Miiller's account C Jenaische Zeitschrift,' 

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



' In a note on p. 50 of his memoir Fiirbringer criticises my description of 

 the mode of growth of the segmental duct. The following is a free trans- 

 lation of what he says : " In Balfour's, as in other descriptions, an account 

 is given of a backward 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 appearance of Dr. Tiirbringer'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 dis- 

 appearance, 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 tliau with any other hiycr. Erom 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 growtli of the duct takes place by wandering cells, but on 

 this point I have no evidence, and must therefore leave the question an 

 open one. — F. M. B. 



