IN THE EMBRYO CHICK. 633 



lost in the adult. The head-kidney remains, however, through 

 life. In Teleostei and Ganoidei (?) the head-kidney is generally 

 believed to remain through life, as the dilated cephalic portion of 

 the kidneys when such is present. In Petr.omyzon and Amphi- 

 bia the head-kidney atrophies. In Elasmobranchii the head- 

 kidney, so far as is known, is absent. 



The development of the segmental duct and head-kidney 

 (when present) is still more important for our purpose than their 

 adult structure. 



In Myxine the development of these structures is not known. 

 In Amphibia and Teleostei it takes place upon the same type, 

 viz. by the conversion of a groove-like invagination of the peri^ 

 toneal epithelium into a canal open in front. The head-kidney 

 is developed from the anterior end of this canal, the opening of 

 which remains in Teleostei single and closes early in embryonic 

 life, but becomes in Amphibia divided into two, three, or four 

 openings. In Elasmobranchii the development is very different. 



" The first trace of the urinary system makes its appearance 

 as a knob springing from the intermediate cell-mass opposite the 

 fifth proto-vertebra. This knob is the rudiment of the abdominal 

 opening of the segmental duct, and from it there grows back- 

 wards to the level of the anus a solid column of cells, which 

 constitutes the rudiment of the segmental duct itself. The knob 

 projects 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 

 appearance in the latter." 



The difference in the development of the segmental duct in 

 the two types (Amphibia and Elasmobranchii) is very im- 

 portant. 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 epi- 

 thelium 1 . 



1 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 translation of what 

 he says : " In Balfour's, as in other descriptions, an account is given of a backward 



B. 41 



