268 



University of California Publications. [Zoology 



The early development of the individual tentacles, whether 

 proximal or distal, includes the same series of chanofes in all, 

 with the one exception that in the latter a slight thickening of 



the ectoderm may precede evagina- 

 tion. Noting this exception, it will 

 not be necessary to describe the 

 course of the development in each 

 set. I have selected as typical of 

 both the development of the proxi- 

 mal tentacle. 



The first changes which I have 

 been able to discover in the origin 

 of a proximal tentacle involve both 

 endoderm and ectoderm. At the 

 l)oint where the tentacle is to ap- 

 l>ear, near the base of the hydranth, 

 from seven to ten epithelial cells, 

 belonging to the lining of the hy- 

 dranth cavity, exhibit certain pe- 

 culiar characters: they are smaller 

 and richer in protoplasm, which 

 may account for their slightly 

 deeper tint when stained with 

 haematoxylin (Fig. 15). I have 

 been unable to determine their ori- 

 gin definitely, except in so far as to 

 be satisfied that they do not spring 

 from interstitial cells. They are 

 either directly transformed func- 

 tional epithelial cells, or products 

 of the division of a few of the lat- 

 ter. In the absence of mitotic fig- 

 ures, both hypotheses might l)e as- 

 sumed to be true. The number of divisions at this stage could 

 at most have been very limited, and might be interpreted as 

 the beginning of the series of divisions which undoubtedly oc- 

 cur in the axial cells of the tentacle after they have been 



Fig. 15. Early stage of 

 proximal tentacle. Optical sec- 

 tion. X 910. 



