12 ARTIFICIAL PARTHENOGENESIS AND FERTILIZATION 



the egg by diffusion, but only if carried into the egg by a living 

 spermatozoon. And when a spermatozoon enters the egg it 

 introduces also into the egg the second substance which pre- 

 vents the disintegration following membrane formation. 



7. The question as to how cytolytic agents cause membrane 

 formation is connected with that of the nature of cytolysis 

 itself. We shall not in this treatise consider the answer to this 

 question, but in order to fix our ideas provisionally, we may 

 assume that the surface of the egg consists of an emulsion 

 whose stability is destroyed by cytolytic agents. This sup- 

 position gives us an explanation of the fact that the eggs 

 of some animals show a slight natural tendency to parthe- 

 nogenesis. In such eggs the stability of the emulsion may 

 be relatively small, so that the HO ions of the sea-water, 

 or the carbonic acid produced by the eggs or by bacteria, 

 are in themselves sufficient to destroy the emulsion and to 

 cause membrane formation. This hypothesis is supported 

 by observations upon the eggs of starfish. Unlike most sea- 

 urchin eggs, those of starfish show occasionally a tendency to 

 develop spontaneously (without the addition of sperm). As 

 a rule one finds that a few starfish eggs begin to divide after 

 lying for some time in sea-water, and in many cases this is 

 followed by the spontaneous development to the larval stage 

 without the necessity for any artificial manipulation. Mathews 

 found that the number of such spontaneously developing 

 starfish eggs can be increased by slight mechanical agitation. 

 I found the same to be the case for the eggs of Amphitrite, a 

 marine worm, which also show a tendency toward spontaneous 

 parthenogenesis. These facts, of which there had hitherto been 

 no explanation, can be understood upon the assumption that 

 this parthenogenetic tendency depends upon the slight stability 

 of the emulsion at the surface of the eggs of these forms. At 

 the lower limit of this stability, a slight shaking is enough to 

 destroy it. I have also found that if the eggs of Asterias are 



