. Analysis of development. Embryology; Embryology. 470 Special Vertebrate Organogenesis which the oviduct grows—or it may also contribute material for its formation. THE GENITAL SYSTEM Most of the structures which make up the embryonic genital system have been taken over from other systems, and their readapta- tion to genital functions is a secondary and relatively late phase in their development. The early differentiation of such structures is therefore independent of sexuality. Also, each embryo is at first morphologically bi- sexual, possessing all necessary structures for the differentiati
. Analysis of development. Embryology; Embryology. 470 Special Vertebrate Organogenesis which the oviduct grows—or it may also contribute material for its formation. THE GENITAL SYSTEM Most of the structures which make up the embryonic genital system have been taken over from other systems, and their readapta- tion to genital functions is a secondary and relatively late phase in their development. The early differentiation of such structures is therefore independent of sexuality. Also, each embryo is at first morphologically bi- sexual, possessing all necessary structures for the differentiation of either sex. The differ- entiation of one set of sex primordia and the gradual involution of the other is normally determined by the sex type of the gonad. The initial problems, then, concern the con- stitution of the gonad primordium and the factors which direct its evolution into an ovary or a testis. STRUCTURE AND ORGANIZATION OF THE EMBRYONIC GONAD The sexually undifferentiated gonad is a composite structure. Male and female po- UNDIFFERENTIATED GONAD. Fig. 177. Diagram illustrating the role of medul- lary (stippled) and cortical components of the un- differentiated amphibian gonad in the differentia- tion of ovary and testis. Small arrows indicate the antagonistic inhibitory action exerted by each com- ponent against the other until final dominance of one is established. tentialities are represented by specific histo- logical elements, medulla and cortex, which have alternative roles in gonadogenesis (Fig. 177). Normal differentiation involves the gradual predominance of one component, while in various types of intersexuality the recessive element persists in varying degree. The extent to which the recessive component develops in the embryo, and the duration of the bisexual phase, differ widely from group to group and greatly influence reversibility under experimental conditions. In amphib- ians, in which both components are well developed over a relatively long per
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Keywords: ., bookcentury1900, bookpublisherphiladelphi, booksubjectembryology
