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University of California Publications. [Zoology 



in the longitudinal section of the tentacle represented in Fig. 

 16 support the view that an incorporation of epithelial cells 

 actually takes place. The presence in the tentacular endoderm 

 of deeply staining granules (Figs. 16 and 17) which, both befon- 



lig. 17. Section through a developing proximal tentacle; very slightly 



oblique. X 960. 



and at a slightly later stage are limited exclusively to the 

 hydranth epithelium, points in the same direction. When the 

 axial endoderm of the tentacle possesses about ten cells, some 

 derived bodily from the hydranth epi- 

 thelium, others possibly products of intra- 

 tentacular divisions, it is sharply set off 

 from the epithelium by the establishment 

 of a delicate layer of intervening meso- 

 gloea (Fig. 18). 



The tentacles, in their development up to 

 this point, may be said to incorporate ele- 

 ments from both ectoderm and endoderm. 

 These elements are similar to each other 

 in either layer. The gland cells are ab- 

 sent which appear in the endoderm when 

 the mouth is established and the animal is prepared to ingest 

 new food materials. Nettle cells, the most conspicuously ditferen- 



Fig. 18. Optical sec- 

 tion, developing distal 

 tentacle. X 175. 



