DEVELOPMENT OF NEPHRIDIA OF PHERETIMA 93 



Once the ' funnel-cell ' reached its dorso-lateral position, 

 it developed into a nephridiiim with a funnel (a septal nephri- 

 dium) ; but it would seem that the terminal duct of the 

 nephridium had, so to speak, lost its original course, having 

 been removed from the body-wall and having been caught in 

 the ' tunnel ' of the septum containing the commissural vessel. 

 The terminal duct followed the course of the vessel and travelled 

 towards the mid-dorsal line, where, on meeting its fellow of the 

 other side, it formed the supra-intestinal excretory duct. 

 It would be a case of induced development and growth, stimu- 

 lated by the course and development of the commissural 

 vessel. When we have once got a septal nephridium formed 

 in the dorso-lateral position, its terminal duct would tend to 

 find a way out. But since the way to the body-wall is blocked, 

 the terminal duct lengthens out and follows the course of the 

 commissural vessel. Examples of this kind of ' dependent 

 differentiation ', a term due to Eoux, are found in the experi- 

 ments of Lewis and Spemann. Lewis has shown that a lens 

 will be formed from any patch of ectoderm taken from some 

 other part of the body and grafted over the optic cup during 

 the development of the eye in Ambly stoma and some 

 species of frogs. Spemann and Lewis have also found that in 

 the absence of contact between the optic cup and the ectoderm 

 the cornea is not developed, that is to say, the overlying 

 ectoderm does not ' clear ' (lose its pigment), it does not thin 

 out, and Descemet's membrane is not formed.^ 



Once the supra-intestinal duct is formed in the mid-dorsal 

 line above gut, the only possible way to discharge the excretory 

 fluid is to have communications with the gut in each segment. 

 This tendency of the nephridial ducts to open into the gut has 

 been noticed in other worms also. Eosa (14) found in one 

 species of Allolobophora (A. antipae) that all the 

 nephridia of the posterior region of the body, instead of opening 

 on the exterior, communicate with a pair of longitudinal 

 canals which open posteriorly into a median vesicle com- 

 municating with the rectum. It would appear that in outline 



^ Jenkinson's ' Experimental Embryology ', pp. 271-7. 



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