DIFFERENTIAL INHIBITION IN OBELIA 493 



6. The surface tension of the dedifferentiated zooid causes 

 the emigrated zooid cells to flow into the stem. In later stages 

 spontaneous pulsations of the stem and of the zooid (these 

 possibly not spontaneous) occur. 



7. Dedifferentiation of the tissues of the tentacles starts at 

 the tip. Progressive histological dedifferentiation of the endo- 

 derm cells can thus be clearly followed in a single section. 



8. Only after the mesoglaea at the base of the tentacle has 

 ruptured can the contents be resorbed (confirming Thacher). 



9. Cnidoblasts with nematocysts can be distinguished within 

 the gastro vascular cavity as resorption proceeds. They may 

 also be seen phagocytosed within large cells, presumably 

 immigrated endoderm cells. 



10. The dedifferentiation is regarded as due to interference 

 with general metabolic processes, and especially with the pro- 

 duction of the energy needed to maintain form against surface- 

 tension. 



11. Eesorption is regarded as the natural result of dedifferen- 

 tiation when there are adjacent cavities into which the cells 

 can migrate. In higher forms it has been largely replaced by 

 phagocytosis. 



EXPLANATION OF PLATE 26. 



Acknowledgements are due to Mr. Chesterman, of the Anatomy Depart- 

 ment, Oxford, for assistance in the preparation of the microphotographs. 



Obelia genie u lata. 



Fig. 1. — Longitudinal section through a normal zooid. X 100. 



Fig. 2. — First stage of resorption, x 115. 



Fig. 3. — First stage of resorption (slightly later than Fig. 2). X 130. 



Fig. 4. — Third stage of resorption, x 146. 



Fig. 5. — Fourth stage of resorption, x 240. 



Fig. 6. — A tentacle of a normal zooid showing the differentiation of the 

 endoderm. x 300. 



Fig. 7. — A tentacle of a zooid in the first stage of resorption showing 

 beginning of dedifferentiation in the distal endoderm cells. Also note 

 especially the reduction in width of the endoderm cells, x 420. 



Fig. 8. — Cells and nematocj'sts in a zooid in the fourth stage of resorp- 

 tion. X 420. 



Fig. 9. — Fusion of tentacles. First stage. X 270. 



