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 period, 

 transformation occurs readily; in amniote 

 embryos, however, development is rapid and 

 the recessive component tends to be vestigial 

 and transient. Under such conditions the 

 state of differentiation at the moment of 

 experimental intervention may be a decisive 

 factor in the result. 



The medullary and cortical componentr 

 of the indifferent gonad are laid down 

 through the activities of the somatic or non- 

 germinal constituents of the genital ridge, 

 conveniently called the structural elements. 

 In the early testis of an amniote embryo 

 (Fig. 178) the medullary component consists 

 of primary sex cords, the cortical element is 

 represented by the regressing germinal epi- 

 thelium; however, as long as this epithelium 

 is present, the testis is potentially bisexual. 

 Conversely, in an ovary the male component 

 is represented by more or less rudimentary 

 medullary cords (primary sex cords) and rete 

 elements. All attempts to control sex differ- 

 entiation experimentally undertake by vari- 

 ous means to influence the development of 

 one sex component at the expense of the 

 other. 



The germinal elements of the gonad are 

 the primordial germ cells, which are often 

 recognizable long before the gonad primor- 

 dium appears. Nevertheless, as will be 

 shown, they are apparently not essential 

 either for the formation of a genital ridge or 

 for the differentiation of specific histological 

 structure. Eventually their fate as gametes 

 depends upon the differentiation of the struc- 

 tural elements and their role in morpho- 

 genesis is secondary. 



EARLY TOPOGRAPHIC LOCALIZATION 

 OF THE GONAD CONSTITUENTS 



In most vertebrates the germ cells and the 

 structural elements of the gonad have dis- 

 tinct and sometimes widely separated origins. 

 Their history, prior to final localization in 

 the genital ridge, may be for the most part 

 considered separately. 



The Structural Elements. In embryos of 

 urodele amphibians, from yolk-plug to mid- 

 dle neurula stages, both structural elements 

 and germ cells are fovmd together in the 

 region of the prospective lateral plate. Heter- 

 oplastic exchange of material from this area 

 at the yolk-plug stage (Fig. 170^) results in 



