390 



stages is e.omposed only of the endodennal cells. Karyokinetic di- 

 vision does not take place, and the closing of the vvalls at the edges 

 is brought about by change in form of the cells in the layer of 

 endoderm. The rapid elongation of the cells of this layer (fig. 2) 

 causes the layer itself to beconie thinner. The endodennal cells accu- 

 mulate along the cut-edges of the wall and form material for the 

 development of the closing membrane. 



In the uext stage (one hour after the opération) the ectoderm 

 of the etlges overgrows the endodermal wall. At the time of the 

 union of the walls the endodermal cells are often still not comple- 

 tely covered by the ectoderm (fig. 4). We can see in this case 

 that the closing membrane is coniposed in the median ]iart only 



Fig. 3. 



Fig. i. 



of the endodermal cylindrical cells. These ectoderm cells 

 a t' t e r w a r d s p r o d u c e the n e w p e r i s a r c. A very narrow 

 layer of this substance covers the closing membrane (fig. 3. 4 np.) 

 and this new perisarc layer is in connection with the old perisarc 

 of the half-cylinder. The fact of the régénération of the perisarc 

 in the pièces eut across was mentioned by Morgan: „A thin 

 perisarc is then secreted over the end". 



Before the union of the walls takes place and before defini- 

 tive formation of the closing membrane, the cells in the centre of 

 the thickened edge become filled with manv granules. The central 

 cells disintegrate and in their place a lumen arises (fig. 3) in which 

 the circulation of the granules appears. The granules are. as was 

 ascertained by Miss Stevens'), „part of the cell - material from 

 the disintegrated" endodermal éléments. Tliis process of disinte- 



') Stevens N. il. Regeneration in Tubularia mesem)iryatithemum. Arch. 

 für Entwiokelung-smeehanili der < Irganismen, Bd. XIU. 19UI. 



