OEIGIN OF THE EXTERNAL OEGANS. 



811 



lida. The tubes of the kidneys in the lower vertebrata are therefore 

 named segmental tiilios, and their common duct (Wolffian), the seg- 

 mental duct. lu the amniota, however, the same correspondence 

 between vertebrate segments and Wolffian body tubes no longer exists. 

 The External Organs. — The existence in the embryo at first of a 

 single outlet or cloaca, for the urogenital passages and the alimentary 

 canal in common, has already been referred to. This condition of the 



Fig. 607. — Development of the Ex- 

 ternal Sexual Organs in the Male 

 AND Female from the Inbifferext 

 Type (from Ecker). 



A, the external sexual organs in an 

 embryo of about nine weelis, in which 

 external sexual distinction is not yet es- 

 tablished, and the cloaca still exists ; B, 

 the same in an embryo somewhat more 

 advanced, and in which, without marked 

 sexual distinction, the anus is now sepa- 

 rated from the urogenital aperture ; C, 

 the same in an embryo of aljoutten weeks, 

 showing the female tj^se ; D, the same 

 in a male embryo somewhat more ad- 

 vanced. Throughout the figures the 

 following indications are employed ; }■)€, 

 common blastema of penis or clitoris ; 

 to the right of these letters in A, the 

 umbilical cord ; p, penis ; c, clitoris ; cl, 

 cloaca ; \({f, urogenital opening ; «, anus ; 



I s, cutaneous elevation which becomes labium or scrotum ; I, labium ; 

 caudal or coccygeal elevation. 



s, sci'otum ; co^ 



parts connected with the surface continues even beyond the time when 

 the sexual distinction has begun to become manifest in the deeper 

 organs, as up to the seventh day in the chick and the end of the eighth 

 week in the human foetus. Previous to this time the cloaca presents 

 itself in the form of a Avide cavity, into the middle of which the intestine 

 descends on the dorsal aspect. The pedicle of the allantois opens by a 

 deep groove or recess anteriorly or on the ventral aspect, and on each 

 side there is a widening, into wliich, in succession from the ventral to 

 the dorsal aspect, open the Mullerian and Wolffian ducts and the 

 ureters. The external opening has the form of a vertical slit wider 

 above and below, and is situated in a raised portion of the common 

 integument, from which all the other parts retire more and more within 

 the cavity of the pelvis as it gradually deepens. 



The first change which takes place in the rudiments of the external 

 organs, and which is common to all embryoes, and therefore to botli 

 sexes, consists in the advance from the sides and behind of the parti- 

 tion which separates the intestinal portion from the rest, thus throw- 

 ing the urogenital ducts into connection with a wide ventral part ot 

 the lower aperture, urogenital sinus, while the intestine is left in com- 

 munication with the narrower dorsal section. The anus, strictly so 

 called, now appears as the opening of the alimentary canal, and in 

 front of it the urogenital aperture forms a narrow vertical slit wider 

 behind than before, and leading into the urogenital sinus. 



In front of the last-named aperture there now rises a well-marked 

 prominence of the integument, the rudiment of the still indifferent 



