Vol. 3] Torrey. — Biological Studies on Corymorpha. 295 



This plastic process of differentiation is accomplished by such 

 means as growth by the absorption of water, mechanical tensions 

 and osmotic pressure. The absorption of water by the larvaZ 

 endoderm accompanies the elongation of the body, which is espe- 

 cially pronounced after the axial parenchyma begins its devel 

 opment and in the region in which it is located, namely, the 

 stem. It is difficult to avoid the suggestion which is thus plainly 

 offered that there is a causal relation between the two facts. 



The alterations in ectoderm and endoderm which establish 

 the boundary between hydranth and stem are optically altera- 

 tions in cell volumes. Especially is the endodermal thickening 

 which develops into the fenestrated membrane initiated by 

 growth processes. The same may be said of the formation of 

 the axial parenchyma of stem and tentacles, as well as the semi- 

 partition of parenchymatous tissue partially separating the pre- 

 and post-tentacular cavities of the hydranth. 



The outgrowth of a plug of endoderm cells in the early de- 

 velopment of tentacle and frustule forces the attenuation of the 

 ectoderm enveloping it. Lateral strains in the region of the 

 developing fenestrated membrane mechanically produced eithei 

 by osmotic pressure of the liquid in the enteron, or the contrac- 

 tion of the body, or both, appear to be responsible largely for its 

 final form. 



The amoeboid movements of the clavate end of a frustula 

 afford the means by which a strain is put upon the organ that 

 attenuates its cells, both endodermal and ectodermal, reduces and 

 finally ruptures its connection with the stem. 



Not only do amoeboid movements of locomotion accomplish 

 form changes. The movements of the tentacles, toward and 

 away from the mouth are correlated with the arrangement of 

 the cells constituting their skeletal axis with their longest diam- 

 eters perpendicular to the plane of movement. These activities, 

 together with the reaction to contact, are thus factors not only 

 in the behavior but in the construction of the polyp. 



Space relations appear unquestionably to exert some influence 

 on the position and numerical development of the tentacles, and 

 on the size and number of peripheral canals. The arrangement 



