38 ADAM SEDGWICK. 



ourselves obliged, for precisely similar reasons to those already- 

 given in the case of the mesonephros, to suppose that that 

 ontogeny is in this respect more primitive in which the duct 

 arises as a continuous groove constricted off from the body 

 cavity than that in which it arises as a solid knob (modified 

 groove) for only a very small part of its course, and undergoing 

 the major part of its early growth quite independently of sur- 

 rounding structure. 



In Elasmobranchii that part which develops as a groove 

 persists as a groove throughout life (abdominal opening of 

 Miillerian duct). 



In Amphibia, &c., that part which develops as a groove 

 becomes constricted off first in the middle, and then backwards 

 and forwards, but in front it is constricted in a manner, 

 according to Tiirbringer not understood, so as to leave the 

 variable numbers of openings of the pronephros. 



However this may be, apparently the openings of the prone- 

 phros develop as unclosed portions of the anterior end of the 

 groove from which the duct arose, and they open into a space 

 placed at the root of the mesentery close to the notochord and 

 close to the point where in a previous stage the body cavity 

 communicated with the muscle plates. 



In the Amphibian, and apparently in the Teleostean, there is 

 no marked structure corresponding to the intermediate cell mass 

 of Elasmobranchii. The muscle-plate cavity is, after its separa- 

 tion from the general body cavity, only separated from the latter 

 by a double layer of cells, forming its ventral wall and the wall 

 of the body cavity ; i. e. there is no portion of the body cavity 

 at first continuous, but subsequently divided up by the coming 

 together of its walls into a series of canals connecting the general 

 body cavity with the muscle plates. 



Now the glomerulus of the pronephros develops in a part of 

 the body cavity anatomically corresponding to the intermediate 

 cell mass of Elasmobranchii, only in Amphibia it does not, in 

 this region, become divided up into chambers corresponding to 

 the segments. 



With this part of the body cavity, from the somatic walls of 

 which the original groove arose, the openings of the head-kidney 

 communicate. The number of these openings corresponds with 

 the number of segments occupied by the pronephros in all those 

 animals in which they exceed one, except Myxine; but the 

 development of the pronephros in Myxine is not at aU known, 

 and its adult structure is, on the whole, obscure. 



Turning again to Elasmobranchs, we find that the anterior 

 knob of the segmental duct arises from the intermediate cell 

 mass, i. e. from a part of the body cavity corresponding serially 



