STUDIES IN ARTIFICIAL PARTHENOGENESIS. 183 



to account for the process, but it is readily understandable 

 on the basis of the surface tension theory. The membrane 

 swelling which occurs at the point of sperm entrance, causes 

 a lowering of surface tension not only in the immediate vicinity, 

 but everywhere on the egg surface. Hence the vitelline mem- 

 brane is lifted from all parts of the egg at practically the same 

 moment. 



G. The Significance of Cortical Change. 



Although it has never been proven that cortical change is 

 absolutely essential before development can take place, it is 

 generally admitted that membrane "formation" must precede 

 any normal development. We- have seen that cortical change 

 or membrane "formation" may involve either of two quite 

 different processes. What then is the fundamental significance 

 of cortical change, why is it necessary for normal development? 

 I think that at least part of the answer is forthcoming. If an 

 egg be watched under a micrometer scale, it will be noticed that 

 shortly before the first cleavage the egg rapidly lengthens in the 

 direction of its polar axes. In three minutes, an egg was in one 

 case observed to increase its polar axis by 6.5/z, in another case 

 by S/JL. (This fact is by no means new, all the pictures of cleavage 

 show an increase in polar axis.) If the egg is surrounded by a 

 stiff membrane, such a rapid change of form could scarcely be 

 possible. But cortical change, whether it be membrane swelling 

 or elevation, always results in the removal of this obstacle. 

 The vitelline membrane is either rendered soft by swelling, or 

 it is lifted away from the egg surface and its place taken by the 

 no doubt less rigid hyaline layer. As a result, rapid changes irt 

 egg form can occur. Moreover, as is well known, the hyaline 

 layer is normally pulled in between the first two blastomeres 

 during the cleavage process. The stiff vitelline membrane 

 could scarcely act in this way. But either membrane swelling 

 or membrane elevation would result in the egg being invested 

 by a membrane which was not too rigid to be pulled in. Thus 

 at least two processes which play a part in normal development 

 would be greatly hindered if some kind of cortical change did 

 not occur. 



