Dimorphism and Regeneration m Metridium. 231 



derived from monoglyphic parents, and nine derived from diglyphic 

 parents. The possible exception mentioned was a diglyphic 

 polyp of diglyphic parentage which had two pairs of directive 

 mesenteries, both apparently in the new tissue. Yet the limits 

 of the old tissue could not in this case be located with certainty, 

 and it is possible that one of the two pairs had really been derived 

 directly from the parent fragment. Otherwise we must suppose 

 that regeneration had taken place in such a way as to produce 

 simultaneously out of new tissue two pairs of directive mesen- 

 teries. That such a thing probably occurs sometimes is indicated 

 by the observation once m a great while of a triglyphic individual, 

 a condition which would be reached if a fragment already con- 

 taining a pair of directive mesenteries acquired two more by 

 regeneration. The triglyphic condition may, however, arise in 

 a different way, viz: by laceration of a diglyphic polyp, which 

 then produces in the area of regeneration a new or third siphono- 

 glyph system. 



It still remains to account for the fact shown in Table II that 

 more diglyphic polyps are produced by digylphic than by mono- 

 glyphic parents. A moment's reflection will show that this is not 

 difficult. If pieces are cut at random from the bases of polyps 

 without reference to the position of the directive mesenteries, it 

 is evident that directives are likely to be included in the fragment 

 removed, twice as often when the parent polyp is diglyphic as when it 

 is monoglyphic, since the diglyphic polyp contains two directive sys- 

 tems on opposite sides of the body, whereas the monoglyphic polyp 

 contains only one. Accordingly we should expect the proportion 

 of diglyphic polyps regenerated to be about twice as great in one 

 case as in the other. The observed proportions are not greatly 

 at variance with this expectation. 



In order to test more fully and directly the hypothesis already 

 presented, — that the condition of a regenerated polyp, whether 

 monoglyphic or diglyphic, depends on whether the parental frag- 

 ment did or did not contain portions of the directive mesenteries, — 

 advantage was taken of the fact that in the experiments summarized 

 in Table II certain fragments had been cut from the bases of 

 parent polyps in such a way as to include a pair of directive 

 mesenteries, and others had been cut in such a way as not to 

 include directives in the fragment removed, the two lots having 

 been reared separately. In the former lot unfortunately the 



