REPRODUCTION 



25 



FIG. 6. Sections through ovary 

 of rat, typical of ovarian structure in 

 mammals. 



tissue in the form of a tubule. Eventually this sac 

 becomes constricted at the surface of the ovary until 

 finally the external opening is entirely closed and the 

 tubule becomes a closed follicle within the tissues of the 

 organ. Within the follicle 

 itself, there have been 

 formed, in the meantime, 

 single, large spherical cells 

 (primordial germ-cells) 

 from which one or some- 

 times two ova are devel- 

 oped. The part of the 

 Graafian follicle not occu- 

 pied by the ovum is filled 

 with a fluid substance. 

 The follicle increases in size and approaches the center of 

 the ovary until near maturity, when it rises to the surface 

 and finally is ruptured, thus liberating the enclosed egg. 

 The bursting of a Graafian follicle and the discharge 

 of the egg is called ovulation. This event is marked 

 by certain phenomena indicating increased sexual activity. 

 It is believed that menstruation or the period of heat in 

 domestic animals is coextensive with the ripening of 

 the egg. It is true, however, that, under some circum- 

 stances, ovulation may occur before or after the period 

 of heat. The ripening of the first Graafian follicle in 

 general marks the beginning of puberty, but has been 

 known to occur even in infancy. In animals generally 

 ovulation does not occur during pregnancy, but there 

 are numerous exceptions to this rule, as will be described 

 later in the case of some animals which have come in heat 

 and have even conceived again, although pregnant at 

 the time (Fig. 7). 



