MODIFICATIONS OF THE REPRODUCTIVE ORGANS. 99 



dition of a process of the ventral wall of the cloaca, grooved 

 on one face. In ornithodelpbous mammals the penial urethra 

 is complete, but open behind, and distinct from the genito- 

 urinary sinus. - In the Didelphiu the penial urethra and gen- 

 ito-urinary sinus are united into one tube, but the corpora 

 cavernosa are not directly attached to the ischium. 



Certain Meptilia possess a pair of eversible copulatory or- 

 gans situated in integumentary sacs, one on each side of the 

 cloaca, but it does not appear in what manner these penes 

 are morphologically related to those of the higher Vertebrata, 



In the female sex, the homologue of a penis frequently 

 makes its appearance as a clitoris, but rarely passes be3'ond the 

 stage of a grooved process with corpora cavernosa and corpus 

 spongiosum — the former attached to the ischium, and the lat- 

 ter developing a glans. But, in some few mammals (e. g., the 

 LemuridcB), the clitoris is traversed by a urethral canal. 



In no vertebrated animal do the ovaries normally leave the 

 abdominal cavity, though they commonly forsake- their primi- 

 tive position, and may descend into the pelvis. But, in many 

 mammals, the testes pass out of the abdomen through the 

 inguinal canal, between the inner and outer tendons of the 

 external oblique muscle, and, covered by a fold of peritonaeum, 

 descend temporarily or permanently into a pouch of the integ- 

 ument — the scrotum. In their com'se they become invested 

 with looped muscular fibres, which constitute the cremaster. 

 The cremaster retracts the testis into the abdominal cavity, or 

 toward it, when, as in the higher mammals, the inguinal canal 

 becomes very much narrowed or altogether obliterated. In 

 most mammals the scrotal sacs lie at the sides of, or behind, 

 the root of the penis, but in the Didelphia the scrotum is sus- 

 pended by a narrow neck in front of the root of the penis. 



In most mammals the penis is enclosed in a sheath of in- 

 tegument, the preputiwn ; and, in many, the septum of the 

 corpora cavernosa is ossified, and gives rise to an os penis. 



In the female the so-called labia majora represent the scro- 

 tal, the la^ia minora the preputial, part of the male organ of 

 copulation. 



Organs not directly connected with reproduction, but in 

 various modes accessory to it, are met with in many Ferfo- 

 Jyrata. Among these may be reckoned the integumentary 

 pouches, in which the young are sheltered during their devel- 

 opment in the male Pipefish {Syngnathus), in some female 

 Amphibia {N'otodelphys, Plpa), and Marsupialia ; together 

 with the mammary glands of the Mammalia. 



