Fig. 22. eg and rg. Th: inv n process begins 

 at the ends or the / . 3 just inside I 



: tips or the end tentacles, the nerve ring 



; invaginated loot or all. Co~p-;oe Fiejs. 23-S7. 

 As development ~o?s on the edges of too grooves grad- 

 olcse ovfr the nerves, the closure taking place 

 in too same order as invagination proceeded; that is,, 

 first over the ends of the radial nerves then fin- 

 ally, after gradually travelling op the radial nerves, 

 closing ov:r the nerve ring. 



By the meeting and subsequent fusion of too 

 edges of the grooves, part of their cavity becomes 

 cut off from th; exterior and io left below the : 

 ous system as th; subneural space. But Ohio will bo 

 refered to again i: -r larva in which the pro- 

 cess of ios formation is mere nearly completed, it 



•on in a few only of the r need 

 larvae of stage "F" , 



The nervous system shows a differentiation 

 into two distinct layers, a fibrous one nearest the 

 water system, and a cellular one lying below the 

 fibers. Figs. 23-20. The nuclei of the cellular lay- 

 er arc oval with their lone; diameter perpendicular 

 to the fibrous layer. 



