184 L. V. HEILBRUNN. 



Moreover when hypertonic solutions are used as reagents, 

 the degree of rigidity of the plasma membrane becomes a factor 

 of importance. As was shown previously (p. 158), exosmosis is 

 favored by a cortical change which tends to soften the vitelline 

 membrane. 



III. INTERNAL CHANGE. THE PROBLEM OF SEGMENTATION 

 A. Theories of Segmentation. 



The real problem in the study of artificial parthenogenesis 

 is not the problem of cortical change, but is rather the analysis 

 of the factors which produce initiation of development. How 

 can we define initiation of development? Although it has been 

 shown that after fertilization there is a quickening of various 

 energetic processes in the sea-urchin egg, plausible as such a 

 view no doubt is, no one has ever shown that all the recorded 

 instances of artificial parthenogenesis involve an increased 

 metabolism. ^ In practically every case, segmentation or mitosis 

 has been regarded as the sole criterion of initiation of development. 

 This is, I believe, wholly justifiable, as the act of segmentation 

 is in itself a beginning of development. 



Of the various theories of artificial parthenogenesis, those 

 of R. Lillie and of Loeb have in recent years met with the most 

 favor. The former believes that the initial change is an increase 

 in permeability of the plasma membrane. This is the direct 

 cause of (i) mitosis, and (2) increased metabolism. In his 

 theory of mitosis (R. Lillie 'u), now widely accepted as the 

 least objectionable of any of the numerous theories of mitosis, 

 he considers the hypothetical plasma membrane as charged by 

 an electrical double layer; this charge is neutralized upon an 

 increase in permeability. Such a neutralization of charge pro- 

 duces, according to Lillie, a lower potential at the surface as 

 compared with the interior, and the difference in potential 

 thus produced brings about a number of internal changes, which 

 lead finally to the formation of a mitotic spindle. I confess 

 that I am unable to understand the fundamental basis of the 

 theory. It appears to me that any drop in the potential at the 

 surface of a body involves simultaneously an exactly equivalent 

 drop of the potential at every point in the interior. This seems 



