EXCRETORY ORGANS. 581 



partially worked out in Acipenser and Lepidosteus*, is on the 

 whole very similar to that in the Teleostei. The first portion of the 



Fig. 397. Tkansverse section through the anterior part of an Acipenser 



EMBRYO. (After Salensky.) 

 llf. medullary groove; Mp. medullary plate; ir//. segmental duct ; C//. notocliord; 

 En. hypoblast; Sgp. mesoblastic somite ; Sp. parietal part of mesoblastic plate. 



system to be formed is the segmental duct. In Lepidosteus this duct 

 is formed as a groove-like invagination of the somatic peritoneal 

 epithelium, precisely as in Teleostei, and shortly afterwards forms a 

 duct lying between the mesoblast and the-epiblast (fig. 396, sg). In 

 Acipenser (Salensky) however it is formed as a solid ridge of the 

 somatic mesoblast, as in Petromyzon and Elasmobranchii (fig. 397, 

 Wg). 



In both forms the ducts unite behind with the cloaca, and a pro- 

 nephros of the Teleostean type appears to be developed. This gland is 

 provided with but one'' peritoneal opening, which together with the 

 glomerulus belonging to it becomes encapsuled in a special section of 

 the body cavity. The opening of the pronephros of Acipenser into 

 this cavity is shewn in fig. 398, pr.?i,. At this early stage of Acipenser 

 (larva of 5 mm.) I could find no glomerulus. 



The mesonephros is formed some distance behind, and some time 

 after the pronephros, both in Acipenser and Lepidosteus, so that in 

 the larvae of both these genera the pronephros is for a considerable 

 period the only excretory organ. In Lepidosteus especially the 

 development of the mesonephros occurs very late. 



The development of the mesonephros has not been worked out in 

 Lepidosteus, but in Acipenser the anterior segmenial tubes become 

 first established as (I believe) solid cords of cells, attached at one 

 extremity to the peritoneal epithelium on each side of the insertion 

 of the mesentery, and extending upwards and outwards round the 

 segmental ductl The posterior segmental tubes arise later than the 

 anterior, and (as far as can be determined from the sections in my 

 possession) they are formed independently of the peritoneal epithe- 

 lium, on the dorsal side of the segmental duct. 



^ Acipenser has been investigated by Furbringer, Salensky, Sedgwick, and also by 

 myself, and LeiDidosteus by W. N. Parker and myself. 



2 I have not fully proved this point, but have never found more than one opening. 



'^ Whether the segmental tubes are formed as ingrowths of the peritoneal epithelium, 

 or hi situ, could not be determined. 



