CORRELATION OF ENDOCRINE GLANDS WITH SEX DIFFERENTIATION 893 



3. General Summary of the Factors Involved in Sex 

 Differentiation in the Vertebrate Group 



The sex glands (gonads) and the reproductive ducts appear to arise inde- 

 pendently of each other. 



The primitive gonad is composed of two main parts: 



( 1 ) the primordial germ cells and 



(2) cellular structures which act as supporting and enveloping structures 

 for the germ cells. 



The presence of the primitive germ cells probably is a primary requisite 

 for the development of a functional reproductive gland (see p. 121). 



In the differentiation of the gonad, two basic sex fields or territories appear 

 to be involved in Tetrapoda and probably also in most fishes. These terri- 

 tories are: 



( 1 ) the medulla or testis-forming territory and 



(2) the cortex or ovary-forming area. 



The sex fields may be controlled by the genes in the autosomal chromo- 

 somes, and there probably is a tendency for one or the other of these fields 

 to be functionally stronger than the other. The heterogametic (XY, XO, ZW 

 or ZO) conditions of the sex chromosomes appear to be associated with the 

 stronger sex field, and the homogametic (i.e., XX or ZZ) combination is 

 associated with the weaker sex field. 



During development, presumably, there is a struggle for supremacy through 

 competition for substrate substances (see Dalcq, '49) by these two sex fields 

 and, under normal conditions, the sex chromosomal mechanism determines 

 which of the two sex fields shall be suppressed and which shall rise to domi- 

 nation. The sex chromosomes thus control the direction of sex differentiation, 

 whereas the field or territory elaborates the power of differentiation. 



Disturbing influences may upset the sex-determining mechanism set forth 

 above, and various degrees of hermaphroditism may arise in the same indi- 

 vidual in proportion to the degree of escape permitted the normally suppressed 

 sex field. 



The sex ducts arise in association with the pronephric kidney and its duct, 

 the pronephric (mesonephric) duct. The Miillerian or female duct arises by a 

 longitudinal splitting of the original pronephric (mesonephric) ducts (e.g., in 

 elasmobranchs) or by an independent caudal growth of a small invagination 

 of the coelomic epithelium at the anterior end of the mesonephric kidney 

 (e.g., reptiles, birds, and mammals). This independent caudal growth is de- 

 pendent, however, upon the pre-existence of the mesonephric duct (Chap. 18). 

 In the urodeles, the Miillerian duct appears to arise partly from an inde- 

 pendent origin and in part from contributions of the mesonephric duct. 



