18 THE INITIATION OF DEVELOPMENT: 



Limnaea suggest that the mutual attraction between egg and 

 sperm nuclei does not arise until both pronuclei are swollen. 

 This swelling itself depends upon a special condition of the 

 egg cytoplasm, a condition which normally is not realised until 

 after the second maturation division, but which may be pre- 



Fig. 5. Egg of a newt, Triton palmatus, constricted before the be- 

 ginning of cleavage (cf. Fig. 13). In the half on the right, the egg 

 nucleus has fused with a sperm nucleus, and cleavage has set in. 

 In the left half, the sperm nuclei farthest removed from the constric- 

 tion are dividing, whereas those closer to it are inhibited. After 



Fankhauser. 



cipitated by external influences (Raven and Roborgh, 1949). 



The penetration of the sperm into the egg thus starts a 

 series of processes in each of which the components of egg 

 and sperm interact, and which result in the initiation of the 

 development of the fertilised egg- Now it is a very remarkable 

 fact that the same result can be achieved along completely 

 different lines. In eggs of many animals, development has been 

 successfully provoked by means of a wide variety of artificial 

 treatments. This is called artificial parthenogenesis. Evidently, 

 the stimulus for development, normally given by the penetrating 

 sperm, is not very specific; very different external stimuli may 

 have the same result. 



In the sea urchin egg, for instance, a method devised by 

 J. Loeb gives very good results. Unfertilised eggs are first 

 treated for a few minutes with a weak solution of butyric acid 

 in sea water. When returned to normal sea water, they im- 



