CHAPTER XLII 



MAMMALIAN DEVELOPMENT 



In mammals the reproduction is entirely sexual and in all of the 

 higher forms the development is intrauterine (within the uterus). 

 With the exception of those of the lowest group, Prototheria, the eggs 

 of mammals bear only a meager amount of yolk. 



The ovaries are small, ovoid, paired organs which are attached one 

 on either side of the wall of the coelom by the supporting mesenteries, 

 mesovaria. The histological structure of the ovary includes the outer 

 {germiTwl) epithelium, the stroma (connnective tissue and few smooth 

 muscle cells), ovarian follicles with ova (eggs) in different stages of 

 development. Fig. 421 shows the origin of a follicle from the germinal 

 epithelium in the formation of an ovigerous tube. Certain cells of the 

 epithelium are the primordial germ cells, which become ova and the 

 adjacent cells form the lining of the primary follicle. The wall of the 

 follicle cells thickens by multiplication of these cells. Soon a cavity 

 forms in this wall of cells as the beginning of antrum or space of the 

 follicle. The space does not entirely surround the developing ovum. 

 The layer of the cells immediately surrrounding the ovum {corona 

 radiata) are joined at one side to the wall of the follicle. The band 

 of cells extending to the wall is known as the cumulus. The outer- 

 most layer of a mature follicle consists of two thin bands of con- 

 nective tissue, the internal theca and external tJieca. The human ovum 

 measures only 0.22 millimeter in diameter and that of a dog is still 

 smaller, measuring only 0.15 millimeter. The follicle becomes dis- 

 tended with fluid (follicular liquor) and at maturity projects beyond 

 the general surface of the ovary. 



The ovum develops in an ovarian (Graafian) follicle. It passes 

 through the stages of oogenesis to the secondary oocyte here (Fig. 

 421). At this point of development the follicle reaches its maturity 

 and its outer wall ruptures to free the maturing ovum. The ruptured 

 follicle from which the ovum has been discharged, immediately fills 

 with a blood clot in which are many of the follicle cells. It later 

 takes on a yellowish color and appears as a mass of epithelial cells. 

 If pregnancy ensues, this body becomes a functional corpus luteum 

 and produces essential hormones, if not, it degenerates to become a 



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