REPROD UCTION. 657 



or partly-dividing eggs. In all the higher mammalia the 

 dentoplasm is relatively sparse and tolerably evenly mingled 

 with the protoplasm, and the whole 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 struc- 

 ture described in the preceding paragraph undergo further 

 changes by which the egg-cell is rendered capable of fertiliza 

 tion. 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 to not quite 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 

 with a little of the cell protoplasm 

 to form a small cell, the first polar 

 globule or directive corpuscle (c, Fig 

 191). The much larger cell result 

 ing from the division and represent- 

 ing the remainder of the vitellus 

 and nucleus now repeats the process, 

 and gives rise to the second polar 

 globule. In Fig. 191 the first polar 

 globule is shown at c. as already 



* FIG. 191. -An ovum about to 



Separated, and the nucleus, d. IS form the second polai globule. 



, . . ,. a, zona pellucida ; b. space filled 



dividing, preparatory to the iorma- with liquid and left by the shrfok- 



, ,i -i" -i , age of the vitellus: c, first polar 



tlOIl OI the Second directive COr- globule; d, nucleus of ovum divid- 



puscle. The stage of karyokmesis 

 is more advanced than those repre- 

 sented m Fig. 10. The two polar of the nuclear spindle, 

 globules lie for a time (Fig. 192) within the zona pellucida 

 in the space left by the shrinkage of the vitellus, but take no 

 part in the formation of the embryo and soon disappear. The 

 rest of the original ovum is now mature and ready for fertili- 

 zation; its nucleus is known as the female pronucleus, fn, 

 Fig. 192. It passes towards the centre of the ovum and forms 



