12. ArRtiricIaL PARTHENOGENESIS AND FERTILIZATION 
the egg by diffusion, but only if carried into the egg by a living 
spermatozoon. And when a spermatozoon enters the ege 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 
