132 PARTHENOGENESIS [CH. 



becomes very small and moves to the edge of the egg near 

 the anterior pole, where there is a thickening of the outer 

 protoplasmic envelope free from yolk. Spermatozoa are 

 received by the female at copulation into the receptaculum 

 seminis, and in some species, for example, the Bees and 

 Wasps, it appears that the female is able to control the 

 fertilisation of the egg; that is to say, she can "at will" 

 allow an egg as it is laid to receive spermatozoa, or withhold 

 them. In other species the females lay equally readily 

 whether they have copulated or are virgin. The maturation 

 process is not affected by the presence or absence of sperma- 

 tozoa; immediately after the egg is laid the egg nucleus 

 breaks down, a spindle is formed vertical to the surface of 

 the egg, and the chromosomes divide and pass to the poles. 

 They may there form daughter nuclei, but more often two 

 new spindles arise immediately, one below the other, with 

 the two groups of chromosomes from the first division in 

 their equatorial planes. These chromosomes then divide 

 again, and pass to the poles, giving rise to four groups in a 

 line vertical to the egg surface. The outermost group is just 

 below the surface, the two middle ones are close together, 

 at the inner end of the outer spindle and the outer end of 

 the inner, and the innermost group, which forms the mature 

 egg nucleus, is rather deep in the egg (cf. Text-fig. 9, p. 8 1). In 

 some species each group forms a vesicular nucleus ; in others 

 the three outer, or polar, groups remain as little clumps of 

 chromosomes, while the innermost sinks inwards and forms 

 the egg nucleus. In either case it is common for the two 

 middle nuclei or chromosome groups to unite; when they 

 are vesicular nuclei they may appear to conjugate just as 

 an egg and a sperm nucleus do, and then break down into 

 chromosomes which may undergo an irregular division. It 

 has been maintained that in the Bee this false zygote-nucleus 



