MATURATION OF THE OVUM. 9 



ova, as in mammals, Amphioxus, echinoderms). It is clear that these conditions 

 of arrangement of the proto- and deuto-plasm within the ovum are the main factors in deter- 

 mining variations in the process of segmentation. 



Maturation of the ovum. Formation of polar globules. Either before 

 its escape from the Graafian follicle, or immediately after, the ovum undergoes a 

 peculiar change, preparatory to, but nevertheless altogether independent of fertiliza- 

 tion, which consists of a process of unequal cell-division or germination, and results 

 in the extrusion from the vitellus of two minute spherical bodies (fig. 8), which have 



Fig. 8. OVUM 07 THE RABBIT FROM THE FALLOPIAN TUBE, TWELVEI 



HOURS AFTER IMPREGNATION. (Bisehoff.) 



On the zona a, spermatozoa are seen, and others in the perivitellinet ." c x. 

 space ; b, the polar globules. 



been termed the polar glooules or directive corpuscles, 

 from a supposition that their presence determines the 

 pole at which the first segmentation will take place 

 should the ovum become fertilized. It is, however, 

 uncertain whether there is any constant relationship of 

 this kind, but it is none the less clear that the extrusion 

 of the polar globules is an event of the highest importance for the due development 

 of the ovum, since until this has happened the ovum appears to be incapable of 

 complete fertilization and segmentation. 1 What is actually extruded is a small part 

 of the nucleus of the ovum, or, to speak more precisely, two small parts of its nucleus 

 in succession, probably surrounded by a very thin investment of protoplasm. Prior 

 to this extrusion, the germinal vesicle approaches the periphery of the vitellus, loses 

 its distinctness of outline, and after passing through phases which are charac- 

 teristic of a nucleus which is about to divide, does actually undergo a division into 

 two, the one part being extruded into a space (perivitelline), which has become 

 formed in consequence of the shrinking or contraction of the ovum, and the other 

 part remaining in the vitellus, only, however, to repeat the process of division, and 

 to form a second extruded globule. The remainder of the germinal vesicle, which is 

 now termed the female pro-nucleus, leaves the periphery of the vitellus for a situation 

 nearer to the centre, where, if fertilization should supervene, it awaits the advent of 

 the male pro-nucleus, which is formed from a spermatozoon. After the two pro- 

 nuclei have come together, a new and complete nucleus is formed by their conju- 

 gation. 



The actual formation of polar globules has not hitherto been observed in the human ovum, 

 although there is no doubt whatever that it takes place. In the rabbit various stages in the 

 process have been traced by E. v. Beneden and Rein, and it has also been noticed in other 

 mammals. But the details of the process have been made outmost precisely (by Fol, Hertwig, 

 and others) in the transparent ova of echinoderms, and more recently and minutely (by 

 3. v. Beneden, Carnoy, Boveri, Zacharias. and others) in Ascaris megalocephala, a thread-worm 

 parasitic in the horse, in which all the changes can be followed in one and the same ovum, 

 or the various phases fixed by means of reagents in different ova, and these may afterwards be 

 stained and studied with the utmost minuteness. The successive changes in such ova are 

 represented in figs. 9 and 10. The polar globules remain visible for a time in the peri- 

 vitelline fluid, and are even seen, should the ovum become fertilized, during the early stages of 

 segmentation, but they ultimately disappear and are not known to take any further part in 

 the subsequent changes which the ovum undergoes. 



The fact that throughout the whole animal kingdom the extrusion of polar globules from 

 the ovum as it becomes mature is almost universal, and that a similar process has also been 

 observed to occur in plants indicates the great importance of the phenomenon. The signifi- 

 cance will be further discussed after the process of fertilization of the ovum has been 

 described. 



1 The ovum may, however, receive a spermatozoon before the completion of the formation of polar 

 globules. 



