FERTILISATION OF THE EGG. 13 



must be regarded as together equivalent to a single ovum. 

 Finally, it is the ovum, not the spermatozoon, that gives rise 

 directly to the embryo : the cases of parthenogenesis seen, for 

 example, in Entomostraca or in the Aphides, show that under 

 certain conditions an egg can develop into an embryo without 

 any participation of spermatozoa, but under no circumstances 

 can a spermatozoon develop into an embryo. 



The act of fertilisation is effected thus. The ripe sperma- 

 tozoa gain access to the ova, either through being introduced 

 into the genital ducts of the female, or, as commonly occurs in 

 aquatic animals, through being discharged by the male over the 

 eggs as soon as these have been laid by the female. By their 

 active swimming movements the spermatozoa quickly make 

 their way to the eggs, bore their way through the vitelline 

 membranes, and so enter the substance of the eggs themselves. 

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

 doubtful whether more than one is ever normally concerned in 

 the process ; indeed, after one spermatozoon has entered an egg, 

 others seem incapable of making their way in. On entering 

 the egg the spermatozoon loses its tail, but its head or 

 nucleus, now spoken of as the male pronucleus, penetrates into 

 the interior of the egg, and makes its way towards the female 

 pronucleus, i.e. the part of the egg nucleus which remains within 

 the egg after extrusion of the polar bodies, for an egg cannot be 

 fertilised until it has ' matured.' The male and female pronuclei 

 rapidly approach each other, meet, and unite to form a single 

 body, the segmentation nucleus. 



The formation of the segmentation nucleus, which completes 

 the act of fertilisation, appears to take place in a very regular 

 and orderly manner, though it is probable that the details are 

 not the same in all cases. The amount of chromatin in the two 

 pronuclei, male and female, is precisely equal in many, though ap- 

 parently not in all cases ; and the arrangement of the chromatin 

 threads during the formation of the segmentation nucleus is a 

 very definite one, so that the male and female threads can be 

 distinguished throughout the whole process. 



The Early Stages of Development of the Embryo. 



After formation of the segmentation nucleus, the develop- 

 ment of the embryo commences almost directly. The earliest 



