2038 



HUMAN ANATOMY. 



Fig. 1716. 



i^orta 



Cardinal vein 



Mesothelium 



Wolffian 

 duel 



Wolffian 

 lubules 



Malpighian body 



Anlage of sexual glands 



Portion of cross-section of early human embryo, showing first appear- 

 ance of sexual glands within germinal ridges. X 60. 



During the development of the Wolffian body, or mesonephros, a second tube, 

 the Miillerian duct, is formed within a linear thickening, the genital ridge, that 

 appears upon the ventro-lateral surface of the Wolffian body. Near the cephalic end 

 of the latter, an e\agination of the lining of the body-cavity into the genital ridge 



occurs, by the contin- 

 ued proliferation and 

 downward growth of 

 the cells of which the 

 evagination is con- 

 verted into a tube — 

 the Miillerian duct. 

 This tube communi- 

 catesdirectly with the 

 body-cavity by means 

 of its trumpet-shaped 

 cephalic extremity, 

 extends parallel with 

 and closely related to 

 the Wolffian duct 

 and, later, below 

 reaches the urogeni- 

 tal sinus. The con- 

 verging lower seg- 

 ments of the two Wolffian and the two Miillerian ducts are embedded within a median 

 mesoblastic band, the genital cord, that represents the continuation of the fused geni- 

 tal ridges of the two sides. Within the genital cord the Miillerian ducts lie in the 

 middle, closely applied to each other, with one Wolffian duct on each side (Fig. 1649). 

 The development of the sexual glands begins about the time that the Miillerian 

 ducts are forming, as a linear thickening of the mesothelium and underlying meso- 

 blastic stroma, situated, however, on the median surface of the Wolffian body ( Fig. 

 1716). Over this raised area, the germinal ridge, the character of the primary peri- 

 toneum changes, its cells becoming taller and undergoing proliferation. Very early 

 among the increasing elements appear specialized cells distinguished by their large 

 size, clear protoplasm, and conspicuous nucleus. These diYeihe pri)nary germ-cells, 

 which later become the primordial ova or sperm-cells, according to sex. For a time 

 this cannot be determined, since in this indifferent stage of the sexual gland special- 

 ization has not yet progressed sufficiently to make differentiation possible. The dis- 

 tinctive features of 



both sexes, there- Fig. 1717- 



fore, are acquired 

 by farther devel- 

 opment of a neutral 

 sex-type in which 

 the indifferent sex- 

 ual glands, the 

 Wolffian tubules, 

 the Wolffian and 

 the Miillerian ducts 

 are the chief com- 

 ponents. In view 

 of the recent in- 

 vestigations on 1-he 

 germ -eel Is, it is 

 probable that the 

 peculiar or sex- 

 chromosomes have much to do with the determination of sex, which differentiation, 

 therefore, dates from the time of fertilization. 



Differentiation of the Male Type. — The development of the testis from the 

 indifferent sexual gland includes the invasion of the proliferated mesothelial cells of 



Pritnary germ-cells 



Proliferating 

 Wolffian stroma 



Germinal 

 epithelium 



Cross-section of germinal ridge of young human embryo, showing 

 early differentiation of primary germ-cells. X 500. 



