108 DEVELOPMENT OF THE FKOQ 



C. Fertilisation of the Egg. 



Fertilisation, or impregnation, consists in fusion of the 

 spermatozoon with the egg ; or, more strictly speaking, fusion 

 of the nuclei of these two bodies. 



The spermatozoa, after being shed over the spawn by the 

 male, swim actively by means of their long tails, penetrate 

 the gelatinous investment of the eggs, bore their way through 

 the vitelline membrane, and so penetrate into the eggs them- 

 selves, which they enter at or close to the upper or black 

 pole. 



A single spermatozoon is sufficient to fertilise an egg, and it 

 is doubtful whether more than one is ever normally concerned 

 in the process. 



In about an hour after the spermatozoon has entered, a 

 pigmented process may be seen projecting inwards from the 

 surface of the egg, with a clear spot in its centre. This spot 

 is the nucleus of the spermatozoon, and is spoken of as the 

 male pronucleus : it penetrates farther into the egg, carrying 

 the pigment with it, so that it appears surrounded by a 

 pigmented capsule connected with the surface of the egg by 

 a pigmented stalk. 



By this time the second polar body has been formed and 

 extruded, and the female pronucleus is the only part of the 

 original egg nucleus still remaining. The male and female 

 pronuclei, which are at first some little distance apart, 

 rapidly approach each other, come into close contact, and 

 after having increased considerably in size, fuse together, 

 about two and a half hours after fertilisation has commenced, 

 to form the segmentation nucleus. 



The segmentation nucleus is a large spherical vesicle im- 

 bedded in finely granular protoplasm, and surrounded by 

 an ill-defined capsule of _ pigment : its formation by the 

 fusion of the .male and female pronuclei completes the act 

 of fertilisation. 



The female pronucleus may be regarded as an imperfect 

 nucleus, and the upshot of the process of fertilisation is the . 

 completion of this nucleus ; the nucleus of the spermatozoon, 

 or male pronucleus, replacing the part of the egg-nucleus 

 which has been lost as the polar bodies. The further 



