285 



number of rings and the proximal and distal segmental tracts are 

 common. 4r instead of 3 r is the most frequent variation. 



A word ought to be said concerning the morphological signification 

 of the unsegmented tract. I have found in Obelia that this tract is 

 often quite markedly segmented, and all degrees in the expression of 

 segmentation exist between a completely smooth, or slightly corrugated 

 surface to so well-marked a segmentation that it is impossible to say 

 what are the exact limits of the unsegmented tract. By counting the 

 number of corrugations in the unsegmented tracts in which the cor- 

 rugations are slightly but definitely marked, I have reached the con- 

 clusion that the unsegmented tract is morphologically equivalent to 

 from two to four rings — 3 on the average. I have, however, counted 

 as many as 10 rings belonging to the unsegmented tract. The fact 

 that the unsegmented tract is potentially composed of rings whose 

 number can either be actually counted or accurately estimated, enables 

 us to substitute the equivalent number of rings for the symbol ut in 

 the formula given above, reducing the whole to the form (p) r + A, 

 in which the coefficient p is the number of rings in the entire stem. 



In order to cut the hydroids , they were placed on a glass or 

 mica plate under a dissecting microscope, and the edge of a sharp 

 scalpel was drawn across one of the youngest stems at the desired 

 level. 



Healthy , active stocks alone were chosen. The cut stock was 

 kept in clean sea-water, usually changed twice a day. Regeneration, 

 including the formation of a young hydranth, takes place in about 48 

 hours, almost the entire growth occurring during the second day. 

 A long latent period thus intervenes. 



A certain proportion of the individuals operated upon failed to 

 regenerate. If the water was not changed during the two days, the 

 percentage regenerated was usually very small. Apart from this, the 

 number regenerated after cuts at the different levels was widely 

 different under equally favorable conditions of water. While regene- 

 ration took place with comparative facility after a cut at the levels 

 ß and y — from 50 °/ to 80 °/ reproducing the lost parts — only 

 about 10 °/ to 40 °/ of the individuals cut at the level a regenerated, 

 in the cases where an actual count was made. 



III. Results. 

 The results of the experiments are given in the following tables, 

 in which the numbers of rings regenerated are given, the estimated 

 number of rings in the unsegmented tracts being included. 



