iS George Tlwiiias Hargitt 



however, whether the increase in surface of ectoderm necessary 

 to permit the growth of the entodermal cohimn to the normal 

 length could be accounted for entirely by the flattening of the 

 cells. There must therefore be proliferation of the ectoderm 

 cells, but whether this is by mitotic or amitotic division or by 

 both could not be definitely determined, though indications sug- 

 gest mitotic division. If at all prevalent, however, it seems rather 

 strange that mitotic figures were not found more abundantly, 

 since sections were made of all stages of development and 

 stained especially to bring out this feature. The occurrence of 

 amitosis may be accounted for partially as an aid in cell pro- 

 liferation, and perhaps also for another purpose to be considered 

 a Httle later. 



In the formation of the entodermal core of the tentacle in the 

 manner shown in figs. 57-60 (i. e.. a single row of cells one on 

 top of another), the question as to the method of development 

 of this condition presents itself. Three explanations may be 

 suggested : 



1. Several cells from the primary evagination may divide and 

 form a double row of cells, as in Tuhularia, and later be forced 

 together to form a single row. 



2. The single cell usually found at the apex of the evaginated 

 fold (figs. 45, 46, 49) may divide, and the cells arising from this 

 division continue to divide until the entodermal core is fully 

 formed. 



3. The cell at the apex of the evagination may mark the be- 

 gining of cell division, but instead of all the cells resulting from 

 this primary division continuing to divide, only the apical cell 

 of the developing entodermal core continues to divide till the 

 core of the tentacle is fully formed. 



That the first will not explain the facts is evident from an ex- 

 amination of sections of different stages, particularly of early 

 stages. The conditions shown in figs. 45. 46, 49, are brought 

 about by the primary evagination as already stated. This con- 

 dition is always foinid at the beginning of the development, and 

 in sections of later stages there is no indication of this double 

 row of cells, except at the base of the tentacle (cf. figs. 570, 



292 



