910 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



exists as a closed, more or less circular follicle, completely separated by 

 the ovarian stroma from the surface (Fig. 414). 



Each compartment, or Graafian follicle, so formed usually contains 

 one, or at the most two, ova, which develop from the large cells 

 (primordial cells) already referred to. 



The lining of the tubule, which it must not be forgotten was derived 

 from the exterior covering membrane of the ovary, is spoken of as the 

 membrana granulosa, and at one point its cells become elevated to form 

 a projection, the proligerous disk, by which the ovum is connected with 

 the membrana granulosa. The follicles are at first only .03 millimeter 

 in diameter, but they gradually become larger, especially at the time of 

 puberty. The entire cavity of the Graafian follicle is occupied by a fluid, 

 the so-called liquor folliculari, which gradually accumulates during the 

 growth of the Graafian follicle. Each Graafian follicle is, therefore, com- 

 posed of an external tunic which is highly vascular in character. Within 

 this is the granular layer rising at one point to form the proligerous 

 disk, in contact with which is the ovum, while the remainder of the fol- 

 licle is occupied with fluid. As the follicles increase in size they 

 become depressed toward the centre of the ovary, but when about to 

 burst again rise to the surface, until, finally, the walls of the Graafian 

 follicle will lie directly below the germinal epithelium on the surface of 

 the ovary. Its diameter is now from one millimeter to 1.5 millimeters. 

 The rupture of the first Graafian follicle corresponds with the occurrence 

 of puberty or of fertility and indicates the period at which the female 

 organism first becomes capable of the functions of generation. The 

 smaller the mammal the earlier does this capability for procreation 

 appear. In the smaller mammals, such as the rabbit, the guinea-pig, and 

 the rat, as well as in birds, it appears within the first year ; in larger 

 animals, as the eat' and the dog, in the second year, while in the largest 

 mammals, as the horse, cattle, and the lion, in three years ; in the llama, 

 in four years ; in the camel, in five years ; in the human female, about 

 the fourteenth year, and in the elephant, between the twentieth and the 

 thirtieth year. 



Puberty in the human female is recognized by the first occurrence 

 of menstruation, which consists in the escape of blood from the external 

 genitals. In warm climates menstruation first sets in between the ages 

 of eleven and twelve years ; in cold climates between the ages of four- 

 teen and sixteen years. The duration of each single menstrual period 

 varies, as a rule, from three to five days, although it may last as long as 

 eight, or only one or two days. Toward the age of forty or fifty 

 years in the human female the procreative faculty disappears and with 

 it the menses cease. Such a period is spoken of as the climacteric or 

 menopause. 



