14 EMBRYOLOGY 



the developing ovary or testis. In the frog the germ cells originate in the roof 

 of the primitive gut. 



In mammals the germ cells are believed to originate directly from the 

 tissues of the developing ovary or testis. As we shall see in our studies on the 

 origin of other structures, such as the adrenal gland, extensive migration of 

 cells does occur. It is very difficult, however, to trace cells from one part of 

 the embryo to another, and thus the very early origin of the germ cells 

 remains something of a mystery. 



It has sometimes been maintained that the number of germ cells of the 

 adult is limited to those which are formed in the early embryo. This does not 

 appear to be true, since some studies of the ovary show that new germ cells 

 continue to form from the tissues of the ovary during each breeding season. 

 Also, if the ovaries are removed, regeneration may occur from adjacent 

 tissues. These facts would lead us to conclude that new germ cells arise dur- 

 ing adult life. As a matter of fact, some embryologists claim that the germ 

 cells which are seen in very early embryonic development do not form the 

 eggs and sperm in the adult but rather degenerate completely and new germ 

 cells arise from the tissues of the ovary or testis. The degeneration of embry- 

 onic cells before they give rise to functional structures does occur. The 

 primitive kidney of the chick embryo, for example, is not functional and 

 degenerates. A permanent functional kidney subsequently develops from 

 other cells. Likewise the blood vessels of the aortic arches degenerate in the 

 embryo. 



Growth of the egg in the ovary 



In vertebrate animals ovulation involves a seasonal or a rhythmic process 

 in which the egg is liberated from the ovary into the body cavity of the 

 female. From here on it is transported to the exterior by means of ciliary 

 movement through channels which we shall consider later. The egg develops 

 in the ovary from a small oogonium to a large egg, which at the time of 

 ovulation may be in one or another of the stages of maturation, depend- 

 ing on the species. This maturation process involves, first of all, a reduction 

 in the number of chromosomes in the egg. With this reduction in the 

 number of chromosomes there is a segregation of the genes, with the result 

 that eggs of different genetic composition are produced within the same 

 species. In addition, in some species there is a segregation of the sex chromo- 



