46 



THE EARLY DEVELOPMENT OF MAMMALS. 



yolk, 4-6 n\ the protoplasmic zone, 10-21 n; the zone of yolk granules, 82-87 fi\ 

 nucleus, 25-27 fi. The corona radiata, cor. r, consists of elongated radiating cells 

 with rounded ends and oval nuclei. The zona pellucida shows a distinct radial 

 striation. This is probably due to the presence of minute canals running through 

 the zona. The ovum proper is separated by a narrow fissure, the perivitelline 

 space, from the zona, within which it lies free and loose. Hence when a fresh 

 specimen is examined, the same side of the ovum, that containing the nucleus 

 and which is the lightest part, is always found uppermost. In this ovum no 



vitelline membrane was observed. The 

 body of the ovum may be divided into 

 an inner kernel containing the yolk 

 granules, and an outer protoplasmal 

 zone, of which the very thin outermost 

 layer is clear and therefore more or less 

 differentiated from the broader, deeper 

 layer, which is granular and constitutes 

 most of the zone, PI. The yolk grains 

 are i //. or less in diameter. They are 

 highly refringent and of various kinds. 

 Their characteristics have not yet been 

 accurately investigated. The nucleus 

 is nearly spherical and has a conspicu- 

 ous nucleolus. In fresh specimens the 

 nucleolus shows amoeboid movements, 

 even at ordinary summer temperatures, 

 for several hours after removal from 

 the ovary. It is only in hardened speci- 

 mens that the reticulum of the nucleus can be clearly observed. 



Fig. 2. — Full-grown Human Ovum before 



Maturation. 

 cor.r. Part of corona radiata. Z, Zona pellucida. 



PI, Protoplasm. Y, Yolk. ««, Nucleus. — 



{After W. Nagel.) 



Ovulation. 



The discharge of the ovum from the ovary is called ovulation. It results 

 from structural changes in the Graafian follicle, and these changes continue after 

 the departure of the ovum, transforming the Graafian follicle into a so-called 

 corpus luteum. The exact history of these changes does not fall within the scope 

 of this work. The essential steps in the process are the growth of the Graafian 

 follicle and the thinning of its wall at a point at the surface of the ovary. The 

 thin part is called the stigma. This breaks through and establishes an opening 

 by which the ovum surrounded by the corona radiata, together with the liquor 

 of the follicle, can escape into the periovarial chamber, whence it makes its way 

 into the Fallopian tube. The growth of tissue in the walls of the collapsed 



