EXCRETORY ORGANS. 705 



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 



if 



FIG. 397- TRANSVERSE SECTION THROUGH THE ANTERIOR PART OF AN ACIPENSER 



EMBRYO. (After Salensky.) 



Rf. medullary groove ; Mp. medullary plate ; Wg. segmental duct ; Ch. noto- 

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

 plate. 



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 

 pronephros of the Teleostean type appears to be developed. 

 This gland is provided with but one 1 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. ^cfi^pr.n. 

 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 segmental 

 tubes become first established as (I believe) solid cords of cells, 

 attached at one extremity to the peritoneal epithelium on each 



1 I have not fully proved this point, but have never found more than one 

 opening. 



B. III. 45 



