Vol. 3] Torrey. — Biological Studies on Corymorpha. 



263 



with perisarc, though tentacles are beginning to push out. 

 Proximal tentacles appear at X in Fig. 12b. One distal tentacle 

 appears at X in Fig. 12a. It is not often that both sets of tenta- 

 cles begin their development so nearly at the same time. One 

 to three, rarely four, distal tentacles usually make their appear- 

 ance, as in Fig. 13, before any of the proximal set. In this 

 figure, the stem is not so abruptly separated from the hydranth 

 externally as in Fig. 12, but this is due 

 mainly to the temporarily shortened and 

 swollen condition of the stem. Inter- 

 nally, the endodermal ridge between the 

 cavities of hydranth and stem is nar- 

 rower, higher and more sharply defined 

 than before; while the endoderm of the 

 hydranth has lost the conspicuous yolk 

 granules so characteristic of the earlier 

 stages. The perisarc usually leaves the 

 hydranth at this time, to be restricted 

 hereafter to a portion of the stem whose 

 upper limit is marked by the annular 

 thickening in the ectoderm. The mouth 

 is formed as the tentacles appear by a 

 break through the thin apical region of 

 the hydranth, and at the stage repre- 

 sented in Fig. 26a, numerous gland cells are present in the endo- 

 derm in its vicinity. 



I wish to call attention here to the significant fact that there 

 is no important increase in the length of the larva until the endo- 

 derm cells begin to vacuolate and increase in size by the absorp- 

 tion of water; further, that the most rapid increase takes place 

 in the stem, where the cells are largest and most highly vacuo- 

 lated. Such a correlation between elongation and absorption of 

 water can hardly be ascribed to accident. It suggests strongly 

 a mechanical morphogenic factor which may be discussed more 

 conveniently and fully in connection with the experimental evi- 

 dence for form-changes in regenerating stems. The subject will 

 be treated, then, in the third paper of this series. 



rig. 11. Optical sec- 

 tion of larva in which 

 hydranth and stem are 

 distinguishable. X 135. 



