256 ARTIFICIAL PARTHENOGENESIS AND FERTILIZATION 



found that a greater or smaller number of them have formed 

 membranes, and have swelled up and cytolyzed. It appears 

 to me that this observation can be explained on the assumption 

 that the pressure destroys in the egg an emulsion that was 

 just near the limits of stability. 



This assumption also explains why occasionally eggs of a 

 starfish may develop "spontaneously." Such eggs probably 

 form a fertilization membrane spontaneously. The case is 

 similar to the spontaneous segmentation of the sea-urchin egg, 

 with this difference, that the sea-urchin egg almost always 

 disintegrates after a mere membrane formation, while some of 

 the starfish eggs can develop without a second treatment. 



Some authors have stated that "any stimulus induces the 

 egg to develop." This declaration is of course incorrect for 

 the sea-urchin egg, and no authority has stated it for this egg; 

 but things of the kind have been credited of the starfish egg. 

 The statement is correct within the limits in which it also holds 

 for the processes of cytolysis. Cytolysis can be induced by 

 very different agencies, including mechanical disruption, cer- 

 tain chemicals, heat, and high and low concentrations of the 

 solution. The reason for this probably consists in the fact that 

 cytolysis consists in the destruction of an emulsion, and that this 

 end can be attained by very different methods. It is, however, 

 obviously as untrue to say that "any stimulus whatsoever" 

 will cause the eggs of the starfish to develop, as it would be to 

 assert that every stimulus will cause cytolysis or haemolysis. 

 It is needless to say that such an assertion also overlooks the 

 role of the second factor in the causation of development. 



The methods of artificial parthenogenesis for the egg of the 

 starfish resemble those of the sea-urchin egg very closely. The 

 main difference between the two types of eggs exists in regard 

 to the necessity of the second factor of fertilization, which does 

 not seem to be required in the case of a small percentage of the 

 eggs of the starfish. 



