S&i THE HUMAN EMBRYO. 



ridges are present as a pair of bands of epithelium, many cells- 

 thick, and lying along the inner sides of the Wolffian bodies. 

 Primitive ova are already present, and, according to Nagel, are 

 found not only in the genital ridges themselves, but also beyond 

 their limits, and especially in the thickened epithelium in 

 the neighbourhood of the Miillerian ducts. This may perhaps; 

 be takfen as an indication that the genital epithelium was 

 originally less sharply circumscribed than at present. 



Nagel has shown that distinct differences may be detected in 

 the genital ridges of the two sexes from as early a period as 

 thirty-two days ; and he is inclined to doubt whether there is 

 absolute identity at any time, even in the earliest stages. 



In the male, the genital ridge, at thirty-three days, shows a 

 fairly definite arrangement of the cells in strings ; these form a 

 network of tortuous anastomosing cords, arranged somewhat 

 regularly, and bound together by connective tissue. Embedded 

 in the cellular cords are the primitive sperm cells. These are 

 comparatively few in number ; their formation ceases at an 

 early stage, in embryos of about six or seven weeks, on the com- 

 pletion of the tunica albuginea ; but in the later stages, although 

 no new primitive sperm cells are formed from the germinal 

 epithelium, those which are already present increase freely by 

 division. The cellular cords themselves become converted into 

 the seminal canals, which are thus derived directly from the 

 germinal epithelium. 



In the female, the primitive ova, in embryos of thirty-three 

 days, are much more numerous than the primitive sperm cells of 

 the male. They are found in various phases of development, and 

 the formation of new primitive ova continues until about the 

 close of gestation. It is very doubtful whether any new primitive 

 ova are formed after birth, and by some authorities their formation 

 is believed to stop about the seventh month. The tendency for 

 the smaller cells to become grouped around the primitive ova, 

 and so form follicles, is evident even in the fifth week, and affords 

 a good clue by which a young ovary may be distinguished from' 

 a young testis, and the sex of the embryo thus determined. 



2. The Accessory Reproductive Organs. 



As in the rabbit, the chick, and indeed the great majority of 

 Vertebrates, the genital ducts of the human embryo are formed 



