THE BIOLOGY OF THE CELL SURFACE 



rating is the point at which the ectoplasm is weakest in that 

 moment. In the zone of membrane-separation it is dis- 

 continuous and shows a break. The normal process of 

 membrane-separation being due to a secretory process is 

 bound up with water movement. As the ectoplasmic 

 material goes into solution, it is washed away. In the 

 normal process just enough water is present for this behav- 

 ior. When the egg is placed in dilute sea-water, the picture 

 is different; the normally occurring recovery in the zone of 

 membrane-separation can not take place, and the ectoplasm 

 breaks down further. Moreover, with the ectoplasm now 

 gone, the endoplasm is without protection and complete 

 cytolysis results. We are thus here dealing with a sensi- 

 tivity due to actual progressive dissolution of colloids in the 

 ectoplasm of the egg. 



The case of the egg of Arbacia is very interesting. Here 

 differences in resistance to dilute sea-water shown by the 

 unfertilized and by the fertilized egg at the time of mem- 

 brane-separation are not so clear-cut. I was, however, 

 able to prove that the period of membrane-separation also 

 in these eggs is a critical one, by studying the swimming 

 larvae reared from eggs treated with dilute sea-water before, 

 during and after membrane-separation.^ Fertilized Arbacia 

 eggs exposed to tap or distilled water before or after mem- 

 brane-separation and then returned to normal sea-water 

 give rise to normal larvae. But eggs exposed to the tap or 

 distilled water during the period of membrane-separation on 

 return to normal sea-water show marked abnormalities in 

 the late stages of gastrulation. There is here, therefore, 

 definite evidence of a differential susceptibility which 

 appears in clear-cut fashion; dilute sea-water exerts a 

 decidedly deleterious action during the process of membrane- 



1 Just, /(jjSc. 



112 



