464 Scientific Proceedings, Royal Dublin Society. 



are still entirely continuous with the ventral (somatic and splanch- 

 nic) mesoblast. Hensen, Spee, and Flemming conjectured that the 

 primitive kidney is itself developed from the epiblast in these 

 Mammals, but of this they produce no direct evidence. It is 

 more probable that the nephridia are of mesoblastic origin, as in 

 other Vertebrates. 



Fig. 1. — Transverse Section of Embryo Rabbit (4 mm. in length, stage of 16 

 somites). [After Flemming.] 



The section is taken just in front of the posterior termination of the intestine. The 

 right side of the figure is the left of the body. There is a small rupture in the left 

 (right of figure) mesoblastic somite, al., mesenteron (intestine) ; cos., coelom 

 (body-cavity); ep., epiblast; hy., hypoblast; i.c.m., intermediate cell-mass; 

 n.c, neural canal; s.d., segmental duct ; som., somatic mesoblast; sp., splanchnic 

 mesoblast. 



Van Wijhe finds that in the ray the pronephros (Yornier) 

 arises, at the commencement of Balfour's stage I., as a continuous 

 evagination from the somatopleur on each side of the body 

 throughout five somites. When the hinder end of this evagina- 

 tion reaches the skin, it fuses therewith, and the place of fusion is 

 the rudiment of the duct of the pronephros (segmental duct). 

 This grows posteriorly, gradually separating from the skin, so 

 that its latest formed end is always fused with it. The meso- 

 nephros (Urnier) is developed shortly after the appearance of the 

 pronephros. 



In the frog Perenyi finds that the duct develops as a canal- 

 like separation from the inner (nervous) cell-layer of the epiblast, 

 which later associates itself with the mesoderm cells of the inter- 

 mediate cell-mass (Grrenzstrang). 



According to the usually-received account, formation of the 

 segmental duct may take place in two ways — (1) either by the 

 closing in of a continuous groove of the somatic peritoneal epi- 

 thelium (Cyclostomi, anterior end only ; Lepidosteus ; Teleostei ; 

 Amphibia) ; or as a solid knob, or rod of cells derived from the 



