THE MORPHOLOGY OF FERTILIZATION 17 



course, in those cases where a sperm aster exists). In di- and tri- 

 spermic eggs the paths of the male pronuclei are clearly curved ; 

 but there is no need to postulate the existence of fancy forces as 

 an explanation. When the pronuclei get near enough to each 

 other, they are pushed apart by their own growing asters. 



The movements of the female pronucleus are not so straight- 

 forward. Fig. 5 shows the various paths followed by the female 

 pronucleus, according to its original position in the unfertilized 

 egg. If these movements are connected with the sperm aster, the 

 influence of the latter must extend far beyond its visible boundaries. 

 While the female pronucleus is moving through the cytoplasm, 

 granules can be seen moving with it and at the same rate ; but once 

 it reaches and starts moving through the sperm aster, always 

 radially with respect to the astral rays, no synchronous granule 

 movements can be observed. The velocity is markedly non- 

 uniform, which is understandable, first because of the inertia of 

 the female pronucleus, which will affect its initial movements, and 

 secondly, because of the gel-like consistency of the sperm aster 

 through which it travels to the male pronucleus. There can be no 

 doubt that the male pronucleus 'attracts' the female pronucleus. 

 This remark is nothing more than the verbal equivalent of what 

 one can see in Fig. 5. If the male pronucleus were not present, the 

 path of the female pronucleus would be different. But there is no 

 implication that the male pronucleus attracts the female pro- 

 nucleus by electrostatic or electromagnetic forces; nor that long 

 range forces, which were almost as popular as hyaluronidase a few 

 years ago, have anything to do with the phenomenon. The male 

 pronucleus might, purely for example, be responsible for cyto- 

 plasmic currents which move the female pronucleus in the re- 

 quired direction. Chambers made the interesting observation that 

 if the female pronucleus happened to be in the centre of the egg, 

 it moved away from this position to enter the sperm aster. He be- 

 lieves that in the initial stages of its movement, the female pro- 

 nucleus is subjected to an attractive stimulus towards the centre 

 of the sperm aster; the direction of this attractive stimulus con- 

 tinually changes as the sperm aster moves towards the middle of 

 the egg. When close to the male pronucleus, Chambers suggests 

 that a further stimulus ('component of force', p. 418) acts on the 

 female pronucleus so that, although approaching the male pro- 

 nucleus, it also travels parallel with it. This second component. 



