392 



neously on both sides. The inner walls (fig. 5. a) of the canals (a) 

 which grow downwards reach the median line, unité and form 

 from the two separate canals a single eavity (fig. 6). During the 

 whole of this process the cijvering ectoderm grows down more 

 slowly than the endoderm. 



The closing membrane extends parallel to the old wall of the 

 stem. It is elear that the stem retains in thèse cases the form of 

 the half-cvlinder even after the complète closing of the eavity 



(fig- 6). ' 



It may be asked, Where does the material come from for 

 the formation of the closing membrane? 



For the formation of the first walls at the edges of the coeno- 

 sarc the material arises from the endodermal laver. After formation 

 of the canal within the wall the further process involves the dis- 



Fig. 6. 



Fi!». 



placement of the ooenosarc in the direction which is indicated in 

 diagram 5. by the arrow. This obiservation accounts also for the 

 fact that in thèse cases, after the formation of the definitive eavity 

 has taken place, the whole coenosarc lies often in the hottom of 

 the half-cylinder. The edges of the perisarc are prominent on both 

 sides of the coenosarc (fig. 6). Karyokinetic division does not take 

 place during the formation of the eavity. 



3. In those cases in which the half-cylinder is very thin and 

 contains very little of the living substance, the endodermal cells 

 accumulate soon after the opération and form several layers over- 

 lying the primitive endoderm (fig. 7). The cells in thèse endodermal 

 layers hâve often very différent forms from those of the primitive 

 endoderm. I think it very probable that this différence of form can 

 be explained by the amoeboid movements which the cells hâve 

 executed. 



In the following stages (1 — 2 hours after the opération) the 



