508 



Bebind segment X the lower portion of the nephrotome soon 

 begins (at a stage with 18 mesodermic somites) to show differentiation, 

 and thus give origin to the coelomic ends of the mesonephric tubules. 



Figs. XIY and XY show how this process differs at the front 

 and hind ends of the mesonephros (in segments XI and XVIII). 



d. ao. 



an. pr. d. 



coel. 



f- 8- 



an. pr. a 



Fig. XL Fig. XII. 



Fig. XI. Transverse section of a Syllium canicula embryo (7 m.ui. long). 

 The section passes through segment VIII, anterior region. 



e.V. cardinal vein; coel. body cavity; d.ao. dorsal aorta; f.g. fore-gut; nej). 

 nephrotome. Other lettering as in Fig. I, 



Fig. XII. Transverse section of a Scylliuni canicula embryo (7 m.m. long). 

 The section is the third behind that represented in Fig. XI. Lettering as in Fig. I. 



The mesonephric tubules arise from a non-segmented "Anlage" in 

 Chrysemys, and thus the mesonephric tubules are not at this stage 

 segmentally arranged. The fact, too, that at the front end of the 

 mesonephros the glomeruli come to be more venti'alJy situated than at 

 the hind end and that the coelomic end of each nephrotome is early 



