1910] Torrey: Regeneration of Hydranth and Holdfast. 2Vt 



them. It can be shown experimentally thai in a regenerating 

 distal segment of the column, after a cut through the Erus- 

 tular zone, the removal of the older frustules accelerates the 

 developmenl of those near the wound; they may lengthen out 

 within a day or two after the operation, when, without the 

 operation their development would have been arrested for many 

 days or weeks. It may he said. then, that the development of a 

 given frustule is conditioned in part by the presence or absence 

 of immature frustules proximal to it. On heteromorphic pieces, 

 as will soon appear in the fourth paper of this scries, frustules 

 may arise in the region between the hydranths, usually in con- 

 nection with a budding process, always after the piece has become 

 attached to the substratum except in certain cases where an 

 injury has been done the column in this region. The appearance 

 of frustules in all such cases is an indication that physiological 

 continuity between the heteromorphic individuals has been, in 

 some decree, interrupted. 



The position in which frustules arise on a segment regener- 

 ating as a single polyp, varies with the size, shape, and differen- 

 tiation of the latter. On the segments of large diameter, they 

 usually appear rather thickly over the rounded aboral end when 

 the cut is transverse; a1 the lower end of the wound, that is. 

 terminally, when the latter is oblique. They may arise singly, or 

 in groups. On the segments of smaller diameter, in which 

 regressive changes toward earlier stages of differentiation are 

 more complete, the frustules may arise terminally or scattered. 

 They reproduce the larval picture much more closely, however, 

 and mass more rapidly in the zone characteristic of the species. 



II. REGENERATION IN THE LARVA. 



A number of experiments were made with larvae possessing 

 5-7 proximal, 4-5 distal tentacles at a period when the tentacles 

 were being rapidly produced under the simplest histological con- 

 ditions. It was expected, accordingly, that regenerative processes 

 would closely approximate the normal development; and this 

 expectation has been realized. A few typical instances may he 

 given. 



A larva with four distal and seven proximal tentacles ( fig. 15 | 



