Artificial Parthenogenesis 107 



develop apparently without any outside "stimulus," 

 and A. P. Mathews found that slight mechanical agita- 

 tion of these eggs in sea water increased the number 

 which developed. It has been shown in numerous 

 experiments by Delage, R. S. Lillie, and the writer, 

 that the substances causing development in the 

 starfish egg are identical or closely related to those 

 which bring about this effect in the egg of the sea ur- 

 chin and in both cases the development is preceded 

 by a membrane formation. 



But how can membrane formation be produced by mere 

 agitation? It seems to me that this can be understood if 

 we suppose that it depends upon the destruction of an 

 emulsion in the cortical layer of the egg. It is conceivable 

 that in the egg of certain forms the stability of this emulsion 

 is so small that mere shaking would be enough to destroy 

 it and thus induce membrane formation and development. 1 



The durability of emulsions varies, and where an emul- 

 sion is very durable shaking has no effect, while where 

 it is at the critical point of separating into two con- 

 tinuous phases a slight shaking will bring about the 

 separation, and where the emulsion is still less durable 

 we observe the phenomenon of a ; ' spontaneous ' 

 parthenogenesis. Eggs like those of most sea urchins 

 belong to the former, eggs like those of some starfish 

 and annelids belong to the second or third type. 



It is impossible to state at present whether the fertil- 



1 Loeb, J., Artificial Parthenogenesis and Fertilization, p. 255. 



