42 CLEAVAGE II 



agent, which is supposed to render the egg immune, or to 

 carry off the toxic substances. 



It must be admitted that theory here leaves us in the lurch. 



It is interesting to notice that the same two phases, de- 

 structive, involving the formation of the membrane, and 

 counteractive, permitting of segmentation and development, 

 can be distinguished in fertilization by a spermatozoon. 

 When the eggs of a Sea-urchin are inseminated with the 

 sperm of a Starfish, they all form membranes. This is due 

 to the contact of the sperm with the egg surface. But while 

 all the sperms touch the egg, they are not all able to enter 

 and complete the process. If so able, then the egg develops 

 normally ; but if the sperm remain outside, the egg under- 

 goes cytolysis, from which, however, it may be saved by 

 timely treatment with hypertonic sea-water. On removal 

 to ordinary sea-water, it develops. The spermatozoon there- 

 fore normally conveys to the egg first the membrane-forming 

 substance and then the counteractor. 



While we still await a more satisfactory explanation of 

 the workings of these stimulants to parthenogenesis, we have 

 been able to gain rather more insight into the mechanism by 

 which the membrane is thrown off. 



The hypothesis adopted by Loeb is based on the fact that 

 the agent employed (a fatty acid) is lipoid-soluble, and upon 

 a certain conception of the structure of the egg-cytoplasm. 

 This structure is supposed to be alveolar, and the contents 

 of the alveoli are supposed to be prevented from coalescing 

 with one another by a coating of a lipoid (lecithin perhaps). 

 The fatty acid destroys this coating, the superficial alveoli 

 coalesce (the fatty acid having only penetrated a short 

 distance below the egg surface), absorb water, and the accu- 

 mulating perivitelline fluid, being hypertonic to sea- water, 

 throws off the surface-layer of the egg inter-alveolar sub- 

 stance as the membrane. 



When the membrane is fully expanded the perivitelline 

 fluid is practically pure sea- water. If there be added to the 

 sea-water a substance which increases the osmotic pressure, 



