THE URO-GENITAL SYSTEM 403 



to the above glands, the function of which is to furnish a 

 liquid vehicle for the spermatozoa, occur certain modified in- 

 tegumental glands, like the preputial, the function of which 

 is to lubricate the parts. 



The external organs of the female are but slightly developed 

 and appear to represent the various elements found in the 

 male, though retained permanently in a reduced and almost 

 embryonic condition. This is best shown by a comparison of 

 the two as they appear in development, differentiating from 

 an indifferent condition common to both, as in the case of the 

 internal parts. As this history begins with a simple cloaca 

 and develops the external parts from its walls and margin, the 

 history recapitulates also, in a very complete fashion, the 

 stages shown phylogenetically in the preceding pages (Fig. 

 114). 



In an early human embryo the cloacal orifice is approxi- 

 mately circular in shape and is surrounded by a rounded and 

 somewhat elevated margin, the genital ridge. From within 

 its ventral wall, and projecting a little beyond the cloacal ori- 

 fice, rises a conical papilla, the genital tubercle [g], which is 

 really in the form of an inverted trough, enclosing the uro- 

 genital sinus and freely open along its ventral aspect, thus 

 forming the genital cleft [r]. At a later stage the cloacal 

 orifice becomes more prolonged dorso-ventrally, and the genital 

 ridge has become more pronounced along the edges, forming 

 two lateral ridges [h~\, instead of a circular lip. The genital 

 tubercle has also developed and projects conspicuously from 

 the ventral margin of the orifice; its groove is still conspicu- 

 ous, but not so widely open, and its lateral lips take on the 

 aspect of rounded folds [c~\ . The terminal end of the rectum 

 has become visible and forms an anus, distinct from the gen- 

 ital parts, but almost continuous with them. 



Thus far the conditions in the two sexes are precisely alike 

 and the stages are termed indifferent, although we have reason 

 to believe that the sex determination is made at a far earlier 

 period than the -first one considered here, probably even in the 

 fertilised egg previous to segmentation. 



