Nature of Formative Stimulatiot 



133 



cytolysis of the whole egg follows. If we take the eggs during 

 this pause (i.e., after the membrane is formed, but before the 



Fia. 39 



Fia. 40 



Fig. 41 



Fig. 42 



Fig. 43 



Figs. 39-43. — Membrane formation and subsequent cytolysis of the sea- 

 urchin egg in a weak solution of saponin in sea-water. Camera drawings from 

 nature. Fig. 39, unfertilized egg at the beginning of the experiment. In this 

 condition the egg was put into sea-water containing a small amount of saponin. 

 The following figures show the changes it underwent in this solution. Fig. 40, 

 membrane formation under the influence of saponin, eight minutes later. If the 

 egg is taken out of the saponln-sea-water in this stage, washed and put into a 

 hypertonic solution for about one half-hour, it will develop into a larva, after it is 

 put back into normal sea-water. If, however, it is left in the saponin solution it 

 imdergoes the rapid cytolysis represented in Figs. 41, 42, and 43. In the above 

 drawing of the egg, cytolysis began at G, Fig. 41, five minutes after the membrane 

 formation. The stages represented in Figs. 42 and 43 were reached a few minutes 

 later. 



cytolysis of the egg occurs) out of the sea-water containing 

 saponin and free them from all traces of saponin by wash- 

 ing them repeatedly with sea-water, they behave as if the 



