REPRODUCTION 515 



fertilized ovum divides to form the first cells of the embryo: 

 such eggs are named holoblastic. 



The Maturation of the Ovum. From time to time, usually at 

 intervals of about four weeks, in a woman of child-bearing age, 

 certain ova after attaining the size and structure described in the 

 preceding paragraph undergo further changes by which the egg- 

 cell is rendered capable of fertilization. These phenomena, known 

 as the maturation of the ovum, result in separation of small parts of 

 the nucleus or germinal vesicle and cell protoplasm from the rest. 

 They are essentially typical cases of indirect cell division (p. 19). 

 The cell-body shrinks a little so as not quite to fill the zona pellu- 

 cida, and the germinal vesicle approaches one side. Meanwhile the 

 nuclear membrane and karyoplasm form the chromatic loop and 

 this divides into the usual two sets of Vs. One set of these, with 

 part of the nuclear plasm, now separates a little of the cell with 

 protoplasm to form a small cell, the first polar globule (c, Fig. 155). 

 The much larger cell resulting from the 

 division and representing the remainder 

 of the vitellus and nucleus now repeats 

 the process, and gives rise to the second 

 polar globule. In Fig. 155 the first 

 polar globule is shown at c, as already 

 separated, and the nucleus, d, is divid- 

 ing, preparatory to the formation of the 

 second one. The stage of karyokinesis 

 is more advanced than those represented FIG. 155. An ovum about 

 in Fig. 10. The two polar globules lie for ^ m ^riESd.ft 

 a time (Fig. 156) within the zona pellu- fP, ac fil i ed l *\ liquid and 



left by the shrinkage of the 



cida in the space left by the shrinkage of vitellus; c, first polar globule; 

 the vitellus, but take no part in the 

 formation of the embryo and soon disap- 



pear. The rest Of the Original OVUm is rangement of its granules near 

 , mm ., . . the end of the nuclear spindle. 



now mature and ready for fertilization; 



its nucleus is known as the female pronucles, fn, Fig. 156. It 

 passes towards the center of the ovum and forms the usual re- 

 ticulum of karyoplasm found in normal resting nuclei (Fig. 8) . 



Ovulation. From puberty, during the whole child-bearing 

 period of life, certain comparatively very large Graafian follicles 

 may nearly always be found either close to the surface of the ovary 



