123 Excretory Or<^ans in J'xiolloslom;!, Sioiiii 



Of ilic nine that are attached the second, third and sixth have small 

 ncplirococls, while in tlic rest the center is made up of non-nucleated 

 protoplasm. The transition hetwecn the nephrotomes represented in 

 Fig. 1 or Figs. 2 to 8 and the one represented in Fig. 10 is gradual. 

 Toward the posterior end the constriction between the nephrotome and 

 the myotome becomes less pronounced than in Fig. 10. It is possible 

 that if this embryo had been allowed to develop nephrotomes would 

 have appeared still farther back; at least they are found farther l)ack 

 in older embryos. 



The only striking difference between the nephrotomes in the anterior 

 region and those represented in Fig. 1 is that in the former the nephro- 

 coels communicate with the splanchnocoel, as may be seen in Fig. 11. 





Kkj. 1(». FlO. ] 1. 



Fig. 10. — Section tliroush the sixty-eighth sesment slunving a uephrotome connected 

 with the myotome, tliough partly separated from it. my, myotome ; Im, lateral meso- 

 blast ; nc, a very small uepliroca>l ; nt, nephrotome ; scl, sclerotome. The myotome, 

 nephrotome and sclerotome are all continuous with one another, and the lateral meso- 

 blast is continuous with the nephrotome. 



Fig. 11.^ — Section showing the nepliroccel, nv, opening into the splanclinoco!], sc. 

 It also shows the way in whicli the splanchnoco?! first appears as small, rounded cavi- 

 ties in the lateral mesoblast. my, the outline of the end of the myotome. 



Here the distinction between the nephrococl and splanchnocoel is at 

 once apparent by the difference in the character of tlieir bounding walls. 

 In this region the nephrotomes come into direct contact with the myo- 

 tomes and in the fifteenth segment on the right side there is actual 

 continuity between the two. 



A few words regarding the distribution of the nephrotomes. In seg- 

 ments nine to twelve there are cavities having the shape and position 

 of nephrocccls, but with bounding walls less well marked than is the 

 case with nephrocoels farther back, and it is doubtful whether thoy 

 would give rise to any part of the excretory system. Nevertheless the 

 feeling can hardly be avoided that these are rudimentary nephrotomes. 

 On the right side in segments thirteen to fifty, and on the left in seg- 

 jnents thirteen to forty-two the neplirotomcs are in direct contact with 



