98 THE ANATOMY OF VERTEBRATED ANIMALS 



distinct posterior and superior aperture, and the opening of a 

 genito-urinary sinus, common to the urinary and reproductive 

 organs, lies in front of it, separated by a more or less consid- 

 erable permmum. 



These conditions of adult Vertehrata repeat the states 

 through which the embryo of the highest vertebrates pass. 

 At a very early stage, an involution of the external integu- 

 ment gives rise to a cloaca, which receives the allantois, the 

 ureters, the Wolffian and Mullerian ducts, in front, and the 

 rectum behind. But, as development advances, the rectal di- 

 vision of the cloaca becomes shut off from the other, and opens 

 by a separate aperture — the definitive mms, which thus ap- 

 pears to be distinct, morphologically, from the anus of an osse 

 ous fish. For a time, the anterior, or genito-urinary part of 

 the cloaca, is, to a certain extent, distinct from the rectal di- 

 vision, though the two have a common termination ; and this 

 condition is repeated in jives, and in ornithodelphous Mam/- 

 malia, where the bladder, the genital ducts, and the ureters, all 

 open separately from the rectum into a genito-urinary sinus. 



In the male sex, as development advances, this genito- 

 urinary sinus becomes elongated, muscular, and surrounded, 

 w4iere the bladder passes into it, by a peculiar gland, the jf^ros- 

 tate. It thus becomes converted into what are termed the 

 fundus, and neck of the bladder, with the prostatic and mem- 

 hranous portions of the urethra. Concomitantly with these 

 changes, a process of the ventral wall of the cloaca makes its 

 ajopearance, and is the rudiment of the intromittent organ, or 

 penis. Peculiar erectile vascular tissue, developed within 

 this body, gives rise to the median corp)US spongiosum and 

 the lateral corpora cavernosa. The penis gradualty protrudes 

 from the cloaca ; and, while the corpus spongiosum terminates 

 the anterior end of it, as the glands, the corpora cavernosa at- 

 tach themselves, posteriorly, to the iscliia. The under, or pos- 

 terior, surface of the penis is, at first, simply grooved ; by de- 

 grees the two sides of the groove unite, and form a complete 

 tube embraced by the corpus spongiosum. The penial urethra 

 is the result. 



Into the posterior part of this penial urethra, which is 

 f^riqucntly dilated into the so-called hidbiis urethrm, glands, 

 called CoiC2Jer''s glands, commonly pour their secretion ; and 

 the penial, membranous, and prostatic portions of the urethra 

 (genito-urinary sinus) uniting into one tube, the male defmtive 

 urethra is finally formed. 



In sundry birds and reptiles, the penis remains in the con* 



