296 



EMBRYOLOGY 



A connection between all the segmental organs is found 

 by Hatschek in Criodrilus also, for they are said to arise 

 from a cord-like thickening of the somatic layer, which 

 extends the whole length of the body dorsal to the ventral 

 longitudinal muscles. These cords are then separated 

 segmentally into loop-like parts, the fundaments of the 

 nephridia. The latter acquire lumina, and open into the 

 body cavity in front of the segment to which they belong 

 through the future funnels, and finally fuse at their posterior 

 ends with the ectodermal wall to form the external openings. 

 The funnels and nephridial ducts arise separately. But 



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Fig. 136.— Diagram of the development of the excretory system of Polygordius 

 (after Hatschek, from Balfoue's Comparative Embryology). 



these statements of Hatschek also find opponents in 

 Vejdovskt and Bergh, according to whom the segmental 

 organs of the Oligochseta arise from separate fundaments 

 by the growth of the cells in the somatic layer and in the 

 partition walls of the primitive segments. Fig. 137 A 

 shows that at the boundary of the septum and somatic 

 layer there is a considerably enlarged cell (tz). It con- 

 tributes especially to the formation of the funnel. Behind 

 it other cells of the somatic layer arrange themselves into 

 a cord of cells (Fig. 137 B), which constitutes the funda- 

 ment of the nephridial duct. In it, as also in the funnel, 

 a lumen makes its appearance later ; the entire structure 



