264 



THE CELL 



it presses between the two synergidae right into the egg-cell. 

 The pollen grain and the pollen tube contain two nuclei, the 

 vegetative one, which takes no part in fertilisation, and the sperm- 

 nucleus. This latter comes to lie at the end of the pollen tube, 

 after this has made its way to the egg-cell ; thence it passes 

 through the weakened cellulose wall into the protoplasm of the 

 egg, whilst two centrosomes advance in front of it ; these latter 

 were discovered by the French investigator, Guignard (Fig. 143). 

 It soon meets the egg-nucleus, which is somewhat larger, and on 

 whose surface also a pair of centrosomes may be distinguished. 



B 



Fig. 144. Egg from Lilium martagon (after Guignard XVI., Fig?. 80 and 81) : A a 

 short time after the union of the egg- and sperm-nuclei ; B a later stage. The fusing of 

 the centrosomes is nearly completed. 



The two nuclei 



Fig. 145. Egg - cell 

 from the embryo- sac of 

 Lilium martagon, with its 

 nucleus undergoing divi- 

 sion. The nuclear plate 

 consists of twenty - four 

 nuclear segments. (After 

 Guignard XVI. Fig. 83.) 



(Fig. 144) then coalesce, as do also the four 

 centrosomes ; these latter unite so as to form 

 two new pairs, of which each is composed 

 of one element of male and one of female 

 origin. The new pairs are situated on op- 

 posite sides of the cleavage nucleus, and 

 there develop into the two centrosomes of 

 the first nuclear spindle (Fig. 145). 



In the same way as in animal sexual cells, 

 the nuclein and the number of nuclear seg- 

 ments derived from it are decreased during 

 the formation of the pollen-cell and of the 

 egg-cell to one half of the quantity present 

 in a normal nucleus. For instance, whilst 

 in Lilium martagon the normal nucleus de- 

 velops during its division 24 nuclear seg- 

 ments which split up into 48 daughter- 



