CLE A FACE AND DIFFERENTIATION 



The evidence that the treatment affects the ectoplasm 

 alone is derived from the following considerations: First, 

 there are no other noticeable or as striking changes elsewhere 

 in the egg. For instance, the time for obtaining blastulae 

 with cuboidal blastomeres and long cilia comes when the egg- 

 surface is breaking down in the process of separating its 

 vitelline membrane. On the other hand, the best time for 

 the production of twins comes when the ectoplasmic surface 

 is being reconstituted, or, one might say, a new surface 

 arises. Now at these moments, separated by a time inter- 

 val of about two minutes, no profound changes either in 

 the egg- or sperm-nucleus or in the cytoplasm below the 

 ectoplasm can be noted. Secondly, the method employed 

 in these simple observations is that of treating the eggs 

 experimentally for fifteen or at the most thirty seconds. 

 An exposure to hypotonic sea-water of this duration is 

 most certainly to be regarded as affecting the egg-surface 

 only. I conclude, therefore, that the various alterations 

 in the development called forth by treating eggs with 

 hypotonic sea-water at intervals after fertilization are due 

 to an effect on the ectoplasm. As development ensues, 

 the ectoplasm undergoes definite changes correlated with 

 the definite periods in development. If these surface 

 reactions are modified, development is modified. I may 

 refer again to the phenomenon of differentiation without 

 cleavage. Certainly this represents an extreme type of 

 developmental modification. As certainly it indicates 

 that the alteration is due to modification in the behavior of 

 the egg-surface. 



Finally I cite an observation on the egg of Asterias^. 

 Normally during early cleavage the blastomeres of this 

 egg lie within the vitelline membrane apart from each 

 other; later, regaining contact they develop into one 

 embryo. When after the second cleavage the four blasto- 



' Just, igjT. 



337 



