1150 



EMBRYOLOGY. 



the appearance of being radially striated, while in some animals (e.g., insects) it 

 presents a small perforation or hole, which is known by the name of the micropyle, 

 and is believed to be the means by which the spermatozoa enter the ovum. 



The phenomena attending the discharge of the ova from the Graafian follicles, 

 since they belong as much or more to the ordinary function of the ovary than to 

 the general subject of the development of the body, are described with the 

 anatomy of the ovaries on a subsequent page. 



Follicnlar cells 

 adhering to zona 

 pellucida. 



Germinal vesicle. 



Zona pellucida 

 or radiata. 



Yolk granules. 



ggp-PriffoOfc 



FIG. 688. Ovum of rabbit. Highly magnified. (After Waldeyer.) 



Maturation of the Ovum previous to Fertilization. Either before or immedi- 

 ately after its escape from the Graafian follicle, important changes take place in the 

 nucleus of the ovum, which result in its partial disappearance and in the forma- 

 tion and extrusion from the yolk of two peculiar bodies, the polar bodies or polar 

 globules of Robin. These changes constitute what is termed the maturation of 

 the ovum, and are preparatory to its being fertilized by the male element or 

 spermatozoon. The nucleus approaches the periphery of the ovum and undergoes 

 the changes associated with karyokinesis ; it then divides into two, and the upper 

 daughter nucleus, with a thin investment of protoplasm, becomes extruded as the 

 first polar body into a space between the yolk and the vitelline membrane, which 

 has been formed in consequence of a contraction or shrinking of the yolk. The 

 lower daughter nucleus undergoes the same process of division, and forms a second 

 polar body, which is in like manner extruded (Fig. 689). The greater part (three- 

 fourths) of the original nucleus is therefore expelled from the yolk in the form of 

 the two polar bodies, and the remaining fourth, which is now called the female 

 pronucleus, recedes toward the centre of the ovum. The shrinking of the vitellus 

 still continues, and a fluid the perivitelline fluid collects in the space between it 

 and the zona pellucida ; in it, spermatozoa, which have passed through the zona 

 pellucida, may sometimes be seen. 



Although the process of maturation has been closely followed in many of the 

 lower animals, it has not yet been successfully demonstrated in mammals. 



It is interesting to note that a similar nuclear reduction occurs in connection 

 with the development of spermatozoa. In the germinal ridge, which is to become 

 the future testicle, certain cells, identical with primitive ova, are found. These 

 are termed spermatoblasts, and they become enlarged to form what are called sper- 

 matocytes, while each spermatocyte ultimately divides into four spermatids. The 

 spermatids become changed, without further subdivision, into spermatozoa, and 





