NOTES ON REGULATION IN STVLAKIA I.ACUSTKIS. 179 



known experiments of Loeb with segmenting Fundulus eggs, in 

 which the cause ascribed was lack of oxygen. 



6. Healing of a cut surface without regeneration. 



The regenerative power is so strong in the asexual naids that 

 it would naturally be expected that regeneration would take place 

 under all circumstances in pieces above a certain limit of size. 

 Certain instances of failure to regenerate have been met with, 

 however, which indicate that the power to regenerate is depen- 

 dent upon certain internal conditions. If an oblique cut be made 

 in front of the pharynx, removing the prostomium and one side 

 of the head segment, including perhaps one eye, the cut surface 

 may frequently heal over and failure to regenerate the prosto- 

 mium may result (Figs. 4 and 5). A number of such cases have 

 been obtained. In some instances the animals were kept for 

 more than three weeks without showing any tendency to regen- 

 erate the prostomium. Since regeneration ordinarily requires 

 only a few days, the effects seemed in these cases to be per- 

 manent. 



Internal conditions favorable to proliferation, such as the ex- 

 posure of cut surfaces of intestine and blood-vessels, are present 

 in nearly all possible experiments with Stylaria. But after section 

 of the prostomium and one side of the head without touching the 

 pharynx, the tendency to proliferate is apparently at a minimum. 

 The ectoderm may, therefore, close over the wound and cut off 

 the outlet for proliferating material. Failure to regenerate 

 seemed to follow only after an oblique cut, but in one instance, 

 after a transverse cut in front of the eyes, a somewhat similar re- 

 sult was obtained. In this animal an outgrowth appeared from 

 the cut surface which was of less diameter than the region be- 

 hind, appearing like a new head segment superimposed upon the 

 old. Its appearance suggested that the outlet for the prolifer- 

 ating material had been narrowed by reduction of the cut sur- 

 face. It is conceivable that still further reduction of the cut 

 surface, or even complete closing of the wound, might occur in 

 case the proliferating tendency was slight (Fig. 3). 



The above cases of failure to regenerate were only occasional 

 instances, showing that the balance between the tendencies to 

 proliferate and to heal over without regeneration is rather deli- 



