OWL AT ION. 



35 



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 ob- 

 served. 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 outer- 

 most layer is clear and therefore more or less differentiated from the broader, deeper 

 layer, which is granular and constitutes most of the zone, PL 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 conspicuous 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 specimens that the reticulum of 

 the nucleus can be clearly observed. 



PL 



Nu. 



MATURATION. 



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

 PI, Protoplasm. Y, Yolk. Nu, 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 de- FIG. 3. FULL-GROWN HUMAN OVUM BEFORE 

 parture 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 Fallo- 

 pian tube. The growth of tissue in the walls of the collapsed Graafian follicle fills 

 up the space of the same, constituting a mass which is known as the corpus luteum 

 on account of its yellow color. The most characteristic elements of this structure 

 are the large cells which contain the pigment. Each cell has a rounded nucleus 

 and a large protoplasmic body, which is also more or less rounded in form. The 

 lutein granules are in these cells. The function of the corpus luteum was long 

 entirely unknown. Recently the theory has been suggested by Born that these 

 cells exert an influence upon the uterus by which it is prepared to receive the 

 ovum. This influence may be suggested to act by means of a chemical substance 

 .(hormone) produced by the lutein cells and added to the blood, which then affects 



