4-'' 



figure 41, 



The cells of tiie apical plate, ap, and those from which, 

 the cerebral ganglia arise, &g, are separated externally by test 

 cells, but internally are, and continue to be, connectel by cells 

 that do not take part in the formation of other organs. 



The other internal cells have increased in number, and 

 their origin can no longer be determned. At a little later 

 stage, figure 42, the dorsal internal cells are arranged in a lay- 

 er, sg, tZaat represents the beginning of the formation of the 

 shell gland. Other cells soon arrange themselves over the re- 

 mainder of the surface inside the test and, together with the 

 shell gland, form a new ectoderm. This ectoderm probably is 

 formed from cells originally surface cells. If this view is 

 true, the original surface cells give rise to at least three 

 groupfi : those tjiat remain at the surface, acquire cilia, and 

 become differentiated as test cells ; those that form the cere- 

 bral ganglia and apical plate ; and those that form the new ec- 

 toderm inside the test. 



In the stage of which figure 42 represents a section, 

 a space, seems always to be present in about the same position. 



This space probably represents the beginning of the liimen 



of the ifiidgut • 



