336 L. V. HEILBRUXX. 



by diminishing the osmotic pressure, would slow osmotic inter- 

 change and if sufficiently great, would prevent it altogether. 



All these points show clearly that the permeability theory of 

 fertilization and artificial parthenogenesis rests on rather doubtful 

 evidence. In Cumingia certainly, there are no facts which 



support it. 



DISCUSSION. 



Bataillon ('12) in discussing the relations between artificial 

 parthenogenesis in amphibia and sea-urchins, states, "II n'y a 

 pas une parthenogenese experimental des Oursins et une des 

 Amphibiens. Ce sont des materiaux differents chez lesquels le 

 rythme des cineses est suspendu et pent etre retabli. S'il y a 

 une Biologic generale, les conditions de 1'arret ont quelque chose 

 de commun, et les conditions de la mise en branle doivent etre 

 comparable." Presumably this is true. The essential factors 

 underlying stimulation to development are very probably alike 

 for every sort of artificial parthenogenesis. It is certain, how- 

 ever, that the subsidiary features of the process are different in 

 each case. It is necessary therefore to study each egg indi- 

 vidually, to determine exactly its physical make-up, and to 

 attempt to discover what changes are significant in producing an 

 initiation of development. This is what I tried to do in the 

 case of Arbacia. 



F. R. Lillie ('19) in referring to my explanation of the process 

 of cortical change in Arbacia points out that this explanation 

 "can hardly apply to other cases where the cortical changes 

 present a different morphological form." This is true and I 

 never intended that it should. As a matter of fact, I clearly 

 recognized that even in the one egg there were two distinct 

 types of cortical change which might be produced by spermato- 

 zoa. The explanation which F. R. Lillie cites was only advanced 

 to cover one of them. At the same time I offered a different 

 explanation for the other. 



F. R. Lillie also objects to my considering cortical change as 

 "a mere epiphenomenon . . . the phenomenon of the primary 

 cortical change is too general to be treated in this fashion and its 

 character in different animal groups is too varied for it to be a 

 mere phenomenon of decrease of surface tension." I must point 



