II CLEAVAGE 39 



The reason for this is that the eggs are not surrounded by 

 a semi-permeable membrane or coating, but that their surface 

 is permeable to these different substances in varying degree. 

 Since then the eggs admit the entrance of the substances to 

 a greater or less extent, the effect may be due in part at least 

 to some other cause than the osmotic pressure of the solution. 

 And indeed the role attributed to these solutions in the later, 

 improved method, in which they are also employed, is 

 different. 



In the improved method the egg is first submitted to the 

 action of an agent which incites the formation of the fertiliza- 

 tion membrane, a feature characteristic of the normal process, 

 but absent, as we have seen, in the earlier experiments. 



The brothers Hertwig had, many years before, caused the 

 formation of this membrane by the use of chloroform. Loeb 

 was thus led to try esters and fatty acids, and of the latter 

 butyric acid was found to be the most reliable. 



The procedure adopted is briefly as follows : the eggs are 

 placed for a short time (from 1-| to 2-| minutes) in sea-water 



containing a little butyric acid (55 c.c. sea- water + 3 c c. 



butyric). They are then removed to sea-water (which must 

 be alkaline), washed, and kept in sea- water (alkaline) for 

 from 15 to 20 minutes. The membrane is now formed in 

 precisely like manner to the normal membrane. Should the 

 eggs remain longer in the sea- water, however, cytolysis occurs; 

 that is, droplets of clear cytoplasm begin to protrude from 

 the surface, are separated off, and this continues until the 

 whole egg is broken up into fragments. To prevent this 

 some corrective must be employed. The eggs are accordingly 

 placed either in a solution hypertonic to sea-water (made 

 usually by the addition of NaCl, for example, 50 c.c. sea- 

 water + 8 c.c. 2-|n NaCl), which must also both be alkaline 

 and contain oxygen, or in a dilute solution of potassium 

 cyanide (for instance, 50 c.c. sea- water + 2 c.c. ^ per cent. 

 KCN). 



The eggs, after remaining some time (half an hour or more) 

 in the corrective solution, are finally retransf erred to sea- water, 



