DEVELOPMENT OF THE CHICK 113 



can retain their vitality for a considerable 

 time, for it is known that hens may lay fertile 

 eggs for three weeks or longer after the last 

 commerce with the male bird. In birds, as 

 well as in other animals in which the egg is 

 large, it is apparently the normal condition 

 for several spermatozoa to enter one egg 

 (polyspermy). In mammals polyspermy is re- 

 garded as abnormal. 



Immediately after the ovum has escaped 

 from the ovarian follicle it is surrounded by 

 a crowd of spermatozoa, a number of which 

 pierce the egg-membrane and enter the germinal 

 disc. What becomes of the tails of the 

 spermatozoa is not known with certainty. 

 The heads, however, enlarge and become 

 sperm-nuclei^ which remain quiescent until 

 the ovum has completed the formation of polar 

 bodies. Then the egg-nucleus approaches the 

 sperm-nucleus and the two fuse to form a 

 common nuclear mass — the first segmentation 

 nucleus — which, after a short resting stage, 

 begins to divide by that process of mitosis 

 commonly followed by animal cells in general. 



Segmentation. — The fertilised egg is morpho- 

 logically a single cell in which the bulk of the 

 protoplasm, with the nucleus, is confined to 



8 



