REPRODUCTION 433 



around it to form the zona granulosa, the whole group con- 

 stituting a Graafian follicle. 



In the testis the groups of cells form seminiferous tubules, 

 in which the spermatozoa or male elements are developed. 



Ovary. In the adult the ovaries are oval structures covered 

 by a columnar germinal epithelium. In the stroma are seen 

 Graafian follicles in different stages of development. The 

 central cell, the ovum, enlarges. The nucleus becomes promi- 

 nent, and the nucleolus is also large. The protoplasm becomes 

 encased in a transparent capsule the zona pellucida. The 

 cells of the zona granulosa multiply, and fluid, the liquor 

 folliculi, appears among them, dividing them into a set attached 

 to the capsule of the follicle and a set surrounding the ovum. 

 When the follicle is ripe it projects on the surface of the ovary, 

 and finally bursts, setting free the ovum into the peritoneal 

 cavity. The ovum escapes into the peritoneal cavity and passes 

 into the trumpet-shaped fimbriated upper end of the Fallopian 

 tube, through which it reaches the uterus. 



Testis. In the adult this is enclosed in a dense fibrous 

 capsule the tunica albuginea. Posteriorly this is thickened, 

 and forms the corpus Highmori. From this processes extend 

 and form a supporting framework. In the spaces are situated 

 the seminiferous tubules, which open into irregular spaces in 

 the corpus Highmori the rete testis, from which the efferent 

 ducts, vasa efferentia, pass away and join together to form the 

 vas deferens. 



In the seminiferous tubules the spermatozoa are produced. 

 Some of the lining cells divide into two, each forming a support- 

 ing cell and a spermatogen. The latter divides and subdivides 

 till a group of cells lie on the top of the supporting cell. These 

 are the spermatoblasts. In each spermatoblast the nucleus 

 elongates and passes to the attached extremity, the protoplasm 

 decreases in amount, and a long cilium develops from the free 

 end, and the spermatozoon is thus produced. 



Semen. When the testes have become active, the glands 

 of the prostate increase and produce a fluid which, with the 

 spermatozoa, forms the semen. 



The (Estrous Cycle. In carnivora and herbivora alike, the 

 adult female has cycles of sexual activity, the oestrous cycles, 

 which are accompanied by changes in the genital organs. The 



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