child] form-regulation in cgelentera and turbellaria 143 



number of authors as the usual methods of regeneration. In case 

 two hydranths are formed, the oral appears earlier than the aboral. 

 The formation of stolons at the aboral end in vigorous pieces, as 

 described above, indicates that the aboral end of the piece is more 

 or less " determined " for stolon-formation. It is probable, there- 

 fore, that the delay in the formation of the aboral hydranth in pieces 

 which have not formed a stolon is due to the time required for the 

 changes preparatory to hydranth-formation instead of stolon-forma- 

 tion. The production of stolons by certain pieces and the production 

 of hydranths from the ends of the stolons when attachment is impos- 

 sible, or much later, after the stolon has become attached, appear 

 to be reactions to unfavorable conditions. Only vigorous pieces 

 form stolons at all ; i. e., in pieces less vigorous the stimulus to 

 hydranth-formation overcomes the stimulus to stolon-formation be- 

 fore the latter becomes effective. Now, in the vigorous pieces which 

 have produced stolons, the stolon itself, when subjected to unfavor- 

 able conditions, develops a hydranth at its tip. Somewhat similar 

 phenomena have been observed in other hydroids. 



An extended study was made of the proportions of both normal 

 and regenerated hydranths from various regions of the stem : this 

 included measurements of regenerating hydranths before their emer- 

 gence from the perisarc, as well as of fully-developed hydranths. At 

 least four different dimensions were measured in each case. Com- 

 parison of the measurements of hydranths from different regions 

 shows that their proportions are characteristically dift"erent. Hy- 

 dranths arising from the extreme oral end of a stem possess different 

 proportions, both before and after emergence, from those arising at 

 the oral end of a piece from the middle or basal portion of the stem. 

 The proportions of aboral hydranths are different from those of oral 

 hydranths, and also differ among themselves, according to the region 

 from which they arise. The form of the hydranth is then not the 

 same under these different conditions of origin : on the contrary, 

 there are characteristic differences corresponding to the different 

 conditions. 



These facts indicate that the various conditions play a part in 

 determining the form of the result in each case. Discussion of their 

 theoretical bearing is reserved until the data can be given in detail. 



In conclusion I desire to express my most sincere thanks to the 

 Smithsonian Institution for the opportunity afforded me of carrying 

 on the work above described. 



Hull Zoological Laboratory, University of Chicago, 

 August, 1903. 



