THE PROBLEM OF ACTIVATION 241 



Loeb (1916) holds that the essential feature in the 

 activation of the egg, whether by fertilization or parthe- 

 nogenesis, is the change underlying membrane forma- 

 tion, which he conceives to be cytolysis of the superficial 

 or cortical layer of the egg. His reason for the latter 

 conclusion is that "all those substances and agencies 

 which are known to cause cytolysis or hemolysis will 

 also induce membrane formation." They are listed as 

 follows : (i) fatty acids ; (2) saponin or solanin or bile salts ; 

 (3) lipoid solvents, e.g., benzol, toluol, ether, chloroform, 

 etc.; (4) bases; (5) hypertonic or hypo tonic solutions; 

 (6) rise in temperature; (7) certain salts, e.g., BaCl 2 , 

 SrCl 2 , NaCNS; (8) the blood serum or cell extracts of 

 certain foreign species. Loeb states that in the case of 

 the sea urchin egg such agents, used so as to restrict 

 the cytolysis to the cortical layer, will cause membrane 

 formation. Rise in temperature has, however, so far 

 been ineffective in the case of the sea urchin egg, though 

 very effective in the starfish and in Nereis. In the case 

 of other eggs again, none of these methods is effective. 



In the sea urchin the development does not proceed 

 to cleavage by action of the single agent, except in the 

 case of hypertonic solutions, but a second agency is 

 required to bring about further development. Hyper- 

 tonic sea-water is the second agent most commonly 

 employed; this when used for the proper length of 

 time insures subsequent normal development. Loeb 

 therefore states that the action of the first agent leaves 

 the egg in a sickly condition, and the action of the 

 second agent is required to save the life of the egg. It 

 is a corrective agent remedying an unavoidable excess 

 of action of the first agent. 



