ii6 FERTILISATION [CH. 



distance into the egg and then stops; there app^rs to be no 

 attraction between it and the egg nucleus at this stage, and 



11 i ^ A i 1"4 . 



it waits until the egg nucleus is ready. As it does so it 

 increases in size until it becomes a large vesicular nucleus, 

 and the chromatin separates out into definite chromosomes 

 which appear in the reduced number. The centrosome, 

 derived from the middle-piece, meanwhile divides, and 

 between the two centrosomes so produced a spindle arises 

 in the neighbourhood of the sperm nucleus. While these 

 changes have been in progress in the spermatozoon, the 

 egg nucleus has been completing its maturation, and when 

 this has happened the mature nucleus shows a definite 

 attraction for the sperm nucleus, and begins to travel 

 towards it. The two nuclei, now alike in size and appearance 

 and each containing the reduced number of chromosomes, 

 thus come together, usually from opposite sides of the 

 spindle formed between the two centrosomes, and $s they 

 come into contact their membranes dissolve and they give 

 rise to two groups of chromosomes, which arrange them- 

 selves in the equator of the spindle. In these cases, there- 

 fore, no complete conjugation of the egg and sperm nuclei 

 as such takes place; the two nuclei break down into groups 

 of chromosomes before uniting, and the intermingling of 

 the paternal and maternal chromosomes takes place only in 

 the anaphase of the first segmentation division/cf. PI. III). 

 In some animals, for example in Copepods, it has been 

 described as taking place even later, for the two groups of 

 chromosomes travel to the poles separately, and these give 

 rise to nuclei which are still recognisably double, so that 

 the paternal and maternal portions of each of the first two 

 segmentation nuclei are still quite distinct (cf. PL XXII, 

 lower series a, b). 



It is clear, however, that the differences between the two 



