v GENEEATIYE SYSTEM OF THE FEMALE 193 



the phanerogams, and the ova of animals, with results which were 

 sufficiently in agreement. We shall limit ourselves to pointing 

 out briefly the principal phenomena observed in ova. Before being 

 expelled from the follicle, or immediately after, the ovum under- 

 goes a preparatory process called maturation which renders it 

 capable of being fertilised. It consists of an unequal karyokinetic 

 division of the ovum cell (gemmation or germination), which leads 

 to the formation of * directive corpuscles or polar bodies, so called 

 because it is supposed that they indicate the point of the pole of 

 the ovum from which starts the cellular segmentation that takes 

 place after fertilisation. While the germinal vesicle dissolves, 



J // /// 



rf 



V VI 



Fio. 72. Formation of polar bodies in ovum of Asterias glacialis. (After O. Hertwig.) 7, the 

 nuclear spindle (sp) has reached the surface of the ovum ; //, at the surface of the ovum a little 

 nipple has formed (rf) which contains half the spindle ; ///, the nipple has separated by con- 

 striction and constitutes a polar body (zk"), while from the interior half of the original nuclear 

 spindle there is developed a complete new spindle (sp) ; IV, the first polar body is raised by the 

 formation of a second nipple (sp) ; V, the second nipple has separated from the ovum to 

 constitute the second polar body (rk) ; VI, the remainder of the second spindle has transformed 

 itself into the nucleus of the ovum (ck). 



there is formed at the expense of its contents a nuclear spindle 

 with two polar radiations at its extremities. This spindle 

 approaches gradually to the pole of the ovum, until it touches the 

 surface of it with its extremities (Fig. 72). At this point of con- 

 tact, the yolk of the ovum rises, and protrudes in the form of a 

 nipple, which afterwards contracts at its base, and finally separates 

 with part of the yolk and half of the spindle, and constitutes a 

 little polar body. The same process repeats itself, after the half 

 of the spindle remaining in the ovum has transformed itself again 

 into a complete spindle ; thus is formed a second little polar body, 

 and the process of maturation of the ovum is complete. 



The semi-spindle remaining in the ovum cell after the emission 

 of the two polar bodies is called the ovarian or female pronucleus. 

 VOL. v o 



