368 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



anlagen as in the mammals. In its simplest form this penis is a thickening of 

 the ventral wall of the cloaca (figs. 390, 391), the upper surface of which bears 

 a longitudinal groove for the passage of the sperm. Below this is a longi- 



FiG. 390. — Ventral cloacal wall and penis of Rhea (schematized), after Boas, b 

 blind sac; /, corpus fibrosum; g, seminal groove; g', its continuation along blind sac; 

 opening of blind sac. Mucous membrane dotted, seminal groove black. 



tudinal band of fibrous tissue (corpus fibrosum), sometimes in places divided 

 into right and left halves. Between this and the surface is cavernous tissue with 

 large spaces which, when filled with blood, render the whole firm and enlarged 



Fig. 391. — Cloacal region of adult turtle {Emys hiiaria), after von Moller. The 

 rectum and cloaca have been laid open from the dorsal surface and the urogenital sinus 

 exposed. From the opening of the sinus into the cloaca a seminal groove extends along 

 the ventral cloacal surface and can be cut off by a pair of folds (plicae urorectales) from 

 the cloacal cavity, os, anal vesicle; b, urinary bladder; 0, opening of anal vesicle into 

 cloaca; p, penis, exserted; pu, plicae urorectales; r, rectum; sg, seminal groove; ug, 

 urogenital groove. 



(erectile tissue). The tip of the structure is produced as a genital prominence 

 which can be traced in mammals as the glans penis. A homologous structure, 

 the clitoris, is developed in the female, where all other parts are lacking. Only 

 a few birds (ostriches, ducks, geese, swans, etc.) have penial structures. 



