EXISTENCE OF HEAD-KIDNEY IN THE EMBRYO CHICK. 13 



ture and development, into some of whicli it is necessary for us 

 to enter. 



The head-kidneyi attains to its highest development in the 

 Marsipobranchii (Myxine, Bdellostoma). It consists of a lon- 

 gitudinal canal, from the ventral side of which numerous tubules 

 pass. These tubules, after considerable subdivision, open by a 

 large number of apertures into the pericardial cavity. From 

 the longitudinal canal a few dorsal diverticula, provided with 

 glomeruli, are given off. In the young the longitudinal canal 

 is continued into the segmental duct; but this connection becomes 

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

 life. InTeleostei 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 Petromyzon 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 backwards 

 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 



^ I am inclined to give up tlie view I formerly expressed with reference 

 to the head-kidney and segmental duct, viz, " that they were to be regarded as 

 the most anterior segmental tube, the peritoneal opening of which had 

 become divided, and which had become prolonged backwards so as to serve 

 as the duct for the posterior segmental tubes," and ■provisioiiallj/ to accept 

 the Gegenbaur-Fiirbriuger view which has been fully worked out and ably 

 argued for by Fiirbringer (loc. cit. p. 96). According to this view the head- 

 kidney and its duct are to be looked on as the primitive and unsegmented 

 part of the excretory system, more or less similar to the excretory system 

 of many Trematodes and unsegmented Vermes. The segmental tubes I re- 

 gard as a truly segmental part of the excretory system acquired subse- 

 quently. — F. M. B. 



