33 ADAM SEDGWICK. 



cells, occupying the position of which, in a still older animal^ 

 Wolffian tubules are seen.^ 



The point I wish to insist upon is that sufficient proof of an 

 actual ingrowth of cell from the peritoneal epithelium has not 

 been given ; but that it is much more probable that the kidney 

 blastema arose in situ, in some cases perhaps in continuity with 

 the peritoneal lining, and in other cases independently of it, 

 but soon becoming united with it to form the nephrostomata. 



The development of the mesonephros in the Amniota has been 

 most fully elucidated in the chick.^ 



In a recent paper I have described the development of the 

 posterior Wolffian tubules from a continuous blastema of cells 

 derived from the intermediate cell mass ; and in the first part 

 of this paper that of the anterior tubules from the cell cords 

 left connecting the Wolffian duct and intermediate cell mass. 



Further, in the chick there is a kind of intermediate method 

 of development of the tubules of the 12th — 15th segments (see 

 above). 



The question here again recurs which was asked before : Are 

 these tubules of the anterior part of the Avian Wolffian body 

 really tubules of the Wolffian body, or have they something 

 to do with the head-kidney ? Por a discussion of this question 

 I must refer below to p. 460. 



TAe MetanepAros. 



In a recent paper^ I have attempted to show that the me- 

 tanephros, which is found only in the Amniota, is developed from 

 a blastema of cells which arises continuously with but behind 

 the blastema from which the Wolffian tubules develop. 



Although the blastema which will give rise to the greater part 

 of the metanephros arises at a comparatively early stage in de- 

 velopment, still it is not till a much later stage that it shifts its 

 position, and begins to show signs of developing into the Wolffian 

 tubules. This late development of the kidney, which in this 

 point to a certain extent resembles the Amphibian mesonephros, 

 is a very remarkable fact. I shall return to it again. 



I have thus run over very rapidly the most salient features 

 in the development of the various parts of the Vertebrate excre- 

 tory system, so far as it is at present known to us. I now turn to 



' Balfour has recently described the existence of solid cords of cells, 

 connected wfth the peritoneal epithelium, in the anterior part of the meso- 

 nephros of the sturgeon (' Comp. Embryology,' vol. ii, p. 581). The 

 origin of these cords is not clear, neither is it certain that they undergo 

 full development. 



' Loc. cit. 



' Loc. cit. 



