THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 415 



they thus give rise to the convoluted seminiferous .tubules. The supporting 

 cells (of Sertoli) are probably derived from the undifferentiated epithelial cells. 



The details of the further development of the spermatogonia to form the 

 the spermatozoa have been described in the Chapter on Maturation. At this 

 point, that is, with the formation of the spermatozoon, the life cycle from a 

 mature male sexual element in an individual to a mature male sexual element 

 in an individual of the succeeding generation is completed. 



The rete cords constitute an anastomosing network of solid cords of small, 

 darkly staining cells, situated in the hilus region. These cords later acquire 

 irregular lumina, which are lined with cuboidal cells, and form the rete testis. 

 Evaginations grow out from the rete and fuse with the ends of the convoluted 

 tubules, thus forming the straight tubules. On the other hand, outgrowths 

 from the rete unite with the tubules in the cephalic portion of the mesonephros, 

 so that a direct communication is established between the convoluted semi- 

 niferous tubules and the mesonephric tubules. There is thus formed the proxi- 

 mal part of the efferent duct system of the testicle (Fig. 365). That portion 

 of the tunica albuginea in which the rete testis lies, becomes somewhat thickened 

 to form the mediastinum testis. 



The stroma of the testicle is derived for the most part from the mesenchyme 

 of the "indifferent" gland or genital ridge. Probably a smaller part is derived 

 from the germinal epithelium (see p. 407). During development, however, 

 the glandular elements increase more rapidly than the stroma, so that in the 

 adult they predominate. There is a tendency for the convoluted tubules to 

 become arranged in groups which are separated by trabecule of connective 

 tissue radiating from the mediastinum. The interstitial cells of the stroma are 

 direct derivatives of the connective tissue cells (Fig. 370). 



Determination of Sex. 



The question of the determination of sex has caused an enormous amount of speculation, 

 and has been the subject of considerable experimental work. The speculation has led to 

 formulation of many hypotheses, some of which have been in part corroborated by experiment 

 and observation, others of which are hypotheses pure and simple. 



The hypotheses may be divided into three groups — according to which the determina- 

 tion of sex is, respectively, progamous, syngamous, or epigamous. The first of these means 

 that the sex of the future individual is determined before the fertilization of the ovum. 

 The differentiation occurs, according to some authors, in the ovum, according to others, 

 in the spermatozoon. Syngamous determination means that the differentiation of the 

 sex of the individual occurs at the time of fertilization. According to the hypotheses of 

 epigamous determination, the fertilized ovum and the young embryo are sexually "indifferent," 

 and the differentiation of sex depends upon external influences acting upon the embryo 

 during its later development. 



There are features of development in certain forms of animals in favor of one or another 

 of these groups of hypotheses. For example, the fact that in one of the Rotifers, in one of the 

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