Ri\i^ciicration in Hydromediisac ly 



EUDENDRIUM RAMOSUM 



Externally the first indication of a developing hydranth is the 

 formation of a knob-like protuberance by the coenosarc. A thin 

 layer of perisarc is secreted around this knob, though this is 

 later ruptured when the hydranth begins to develop further and 

 the tentacles to form. This protuberance increases in size, elon- 

 gating more or less and assuming a spherical shape. Then the 

 tentacles bud off; at first very small, they later assume their nor- 

 mal length by new growth. The proboscis seems to form as a 

 swelling or evagination at the distal end of the hydranth. and 

 rather late in the development the mouth opening breaks through. 



Transverse sections of very early stages ot the regenerating hy- 

 dranths, i. e., sections through the knob-like structure previously 

 referred to, show the layers of ectoderm and entoderm to be much 

 wider than in the normal stem regions, the cells being extremely 

 elongated radially (fig. 44). There are more cells present than 

 in the normal stem, so that there must have been considerable cell 

 proliferation. Indications of mitosis were found in both ectoderm 

 and entoderm, though not as abundant as ill the entoderm of later 

 stages. In the latter, however, mitoses were far from common. 

 Indications of amitotic division were observed, being quite marked 

 in some cases. 



The tentacles seem to start by evagination involving both lay- 

 ers, and somewhat similar to the same condition in Tiihnlaria 

 tenella (figs. 45 to 47). The further development of the tentacles 

 takes place by new growth. The entoderm cells are arranged in 

 a single row, being superimposed one upon another (figs. 57a, 

 57&). These entoderm cells were sometimes found in the process 

 of dividing mitotically, and this phenomenon was occasionally ob- 

 served in the ectoderm cells surrounding the tentacles. Amitotic 

 division was not common among them, though occasionally found. 

 The shape of the cells, however, was changed from columnar to 

 cuboidal. Sometimes they were flattened even more (fig. 60), 

 so that their length radially was less than their dimensions in 

 other directions. This flattening of the ectoderm cells would in- 

 crease the surface to a considerable extent and thus allow growth 

 of the entodermal core of the tentacle. It is extremely doubtful, 



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