THE PROCESS OF FERTILIZATION 295 



without the aid of artificial staining. The nuclear membrane breaks 

 up ; the centrosome of the ovum, which, although hardly visible, had 

 previously lain beside the germinal vesicle, divides into two centrosomes 

 and their centrospheres, and these now form the ' mitotic figure ' by 

 moving away from each other and sending out their protoplasmic rays. 

 This nuclear spindle soon ranges itself at right angles to the surface 

 of the egg, which at the same time arches itself into a protuberance, 

 and soon two daughter-nuclei are formed, one of them lying within 

 the protuberance (Fig. 75, A, Rki). This soon separates itself off from 

 the ovum, surrounded hy a small quantity of cell-substance. The 

 other daughter-nucleus remains within the ovum, but neither of them 

 remains in a state of rest ; both are again transformed into a spindle 

 and divide once more ; the minute first ' polar body ' dividing into two 

 'secondary polar bodies' of half the size {B, Eki), wdiile the nuclear 

 spindle within the egg brings about a second division of the ovum 

 [B, Rk2) whose unequal products are the second polar cell and the 

 mature ovum^that is, the ovum ready for fertilization. The process 

 is now complete ; the egg-cell, which has lost very little plasmic 

 material through the ' polar bodies ' and has not become visibly 

 smaller, has now a nucleus {B, Elk) which has become considerably 

 smaller through the two rapidly successive divisions, and, as we shall 

 see later, has also undergone internal changes. In this state it is 

 ' ripe,' that is, it is ready to enter into conjugation with the nucleus of 

 a male cell, and this we have already recognized as the essential 

 element in the process of fertilization. 



These processes of ' maturation of the ovum ' are common to all 

 animal ova which require fertilization, and they follow almost the 

 same course, only that in many cases the second division of the first 

 polar body does not take place, so that only two polar bodies in all 

 are formed. All these processes have nothing directly to do with 

 fertilization, but it is only through them that the ovum becomes 

 capable of fertilization. This does not prevent the spermatozoon from 

 previously making its way into the ovum, for this is usually the case 

 (Fig. 75, A, sjj) ; there it waits until the second 'directive division' of 

 the ovum has been accomplished, utilizing the time to become trans- 

 formed in the manner necessary for the conjugation of the two nuclei. 

 Only in a few species, for example in the sea-urchin, does the egg 

 complete its polar divisions within the ovary, therefore before it has 

 come into contact with the sperm at all. 



That we may be able to penetrate still more deeply into the 

 processes of fertilization, the best illustration to take seems to me to 

 be, as yet, the ovum of the thread-worm of the horse {Ascaris 



