DEVELOPMENT OF NEPHRIDIA OF PHERETIMA 



61 



In the second stage that follows we have the appearance and 

 development of the primary pair of septal nephridia in each 



Text-fig. 1. 



SJITT- 



Diagrammatic representation of the three stages of development 

 of the nephridial system in Pheretima. A represents a 

 diagrammatic section of an embryo about 4 mm. in length, 

 showing the paired condition of nephridia (meganephric stage). 

 B represents a stage at which the embryo has two pairs of 

 nephridia, in each segment, a primary integumentary pair of the 

 first stage and a primary septal pair. C shows the formation 

 of secondary septal and integumentary nephridia. In B and 

 C the intersegmental septum is shown on the left half, h.w., 

 body-wall ; g., gut ; d.v., dorsal blood-vessel ; v.v., ventral 

 blood-vessel ; s.n.v., subneural vessel ; n.c, nerve-cord ; p.i.n., 

 primary integumental nephridia ; p.s.n., primary septal 

 nephridia ; s.i.n., secondary integumentary nephridia ; s.s.n., 

 secondary septal nephridia. 



segment of the body behind the first fourteen. As these 

 nephridia begin to appear before all the integumentary ones 

 of the first stage have attained to their full development and 



F 2 



I 



