391 



gration appears in the endodermal oells of the wall and shows 

 where the pi'eliminary circulation takes place, although the phy- 

 sidlogical importance of this circulation is not clear to me. 



2. When two pièces (half-cylinders), obtained by longitudinal 

 cutting, are almost equal. the course of the closing of the eavity 

 is the following. The process begins as in the case just described. 

 We can see in the living material the thickening of the edges. 

 After three or four hours a circulation of the interior fluid begins 

 within the tliickened edges. The impression is produced that each 

 thickened edge contains a canal in which the circulation takes 

 place. The further process is différent froni that in the former case. 

 The walls do not grow towai'ds one another forming a cross bridge 

 over the old eavity. but we see hère a graduai thickening of the 

 whole old wall. Simultaneously a widening of the inner canal of 

 the walls takes place. The fluid of the canal contains an ever 

 increasing number of globules, and the circulation Ijecomes faster. 

 Six or seven hours after uperating, the two thickenings which take 

 place simultaneously on both sides come together in the median 

 line of the bottom of the half- cylinder. forming hv the union of 

 the two canals a single eavity. The entire process may be observed 

 in the living material. 



Sections of the earliest stages show that the endodermal 

 cells accuinulate along the whole length 

 of the edges. The new endodermal walls 

 are soon afterwards overgrown by the 

 ectoderm. In the following stages the cells 

 in the center of the new walls disintegrate 

 and give rise to the first globales which 

 we see afterwards in cii'culation. In place 

 of the disintegi'ated cells a kind of canal 



is formed in which the circulation of the fluid and çjranules begins. 

 Later the innermost wall (fîg. 5. ai of the canal extends downwards 

 parallel to the old wall of the half - cylinder (in diagram 5. the 

 direction of downgrowth is indicated by the arrow) and the wide- 

 ning of the canal is thus effected. 



The direction of the growth of this closing membrane is hère 

 différent /rom the former case in which the folds hâve grown 

 across towards another forming a bridge ovér the old eavity. The 

 downgrowth of the closing membrane takes place hère simulta- 



Fig. 5, 



