122 SEGMENTATION [CH. 



nuclei depends on the region of the outer protoplasm with 

 which it happens to be associated when it comes to the 

 surface at the formation of the blastoderm. 



One other point of some interest in connection with 

 cleavage remains to be mentioned. In developing eggs in 

 which the blastomeres are all approximately equal in size, 

 all the cells divide simultaneously. When there is much yolk 

 in some of the cells, or after visible differentiation into cells 

 of different sizes has begun, the larger cells begin to lag 

 behind the smaller ones, but in some eggs, even many hours 

 after segmentation has begun, the simultaneity of division 

 is very striking. In Insects, for example, it persists until 

 the blastoderm stage ; an egg may often be found in which 

 all the cells of the blastoderm are not only dividing, but are 

 in the same stage of division. This may be due simply to 

 the fact that all the cells, being similar, develop at the 

 same rate, but the possibility of mutual influence upon one 

 another must not be overlooked. That the blastomeres do 

 influence one another is clearly shown by the fact that in 

 an equipotential egg each of the first two or four cleavage 

 cells, if isolated, will produce a complete embryo, though 

 when associated with its fellows it gives rise only to one 

 half or one quarter. The association and mutual position of 

 the blastomeres is thus an essential condition for the 

 differentiation of parts of the embryo. 



Although not very nearly connected with the preceding 

 subjects, it will be convenient to give here some account of 

 the phenomenon of polyspermy, that is, of the abnormal 

 fertilisation of the egg by two or more spermatozoa, to 

 which reference has already been made in earlier chapters. 

 In Echinoid eggs the entrance of a spermatozoon normally 

 not only induces the almost instantaneous secretion of a 

 "fertilisation membrane," but also very quickly renders 



