Regeneration in Hydromeditsae 19 



Syb). If a double row of cells formed by cell division and later 

 these were forced into a single row, the double row would un- 

 doubtedly still be present at early stages. That this is not the 

 case is shown by a comparison of fig. 57a. In this tentacle the 

 entoderm is composed of only four cells, and these are arranged 

 in a single row. This was always the condition after the ten- 

 tacle had begun to elongate. It was more difficult to determine 

 which of the other two suggested explanations was the more 

 probable, because of the paucity of definite mitotic figures. The 

 following facts, however, seem to point strongly to the second 

 as the more probable explanation. In fig. ^yb at "a" are shown 

 two cells somewhat smaller than the others of the entodermal 

 column. These two cells have every appearance of being the 

 products of the division of a single large cell situated in that 

 region, i. e., they indicate a division of one of the middle cells 

 of the entodermal column. Furthermore, in transverse sections 

 through about the middle portions of the developing tentacles, 

 mitotic figures were found in the entoderm, which would not 

 be the case if only the apical cell divided. Comparisons of some 

 of the longitudinal sections show that the entoderm cells in the 

 lower (proximal) end of the core of the tentacle are not of the 

 same size, as would be the case if only the apical cell divided, 

 some are large and some are small as though the result of inter- 

 calary division. These facts and observations show quite con- 

 clusively that the entodermal core of the tentacle is the result 

 of the division of the single apical cell of the primary evagina- 

 tion, and then a continued division of all of the cells resulting from 

 the first division. The entodermal core is surrounded by the 

 ectoderm in early stages (figs. 48, 50), and in the further de- 

 velopment stretches the ectoderm by the division of the cells of 

 the ectoderm (figs. 57a, S7b, 60). The increase in surface of the 

 ectoderm is therefore partly due to the change in form of the 

 tells from a columnar condition to a more or less flattened con- 

 dition, though there is doubtless some cell division in the ecto- 

 derm also. Figs. 52-56 show the course of development of two 

 tentacles from the early stage in which a single ectoderm cell 

 has been forced away from the enteric cavity, to the condition 



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