622 COMPARATIVE ANATOMY. 



placed below tlio two fibrous ones) it forms an erectile welt, wliich 

 represents a penis. Special muscles, which, are inserted into the 

 fibrous bodies, act as retractors of the penis ; in Struthio this 

 organ is provided with special elevator muscles, and is hidden in a 

 diverticulum of the cloaca. 



The copulatory organs of the Mammalia also belong to the second 

 type ; those of the Monotremata differ markedly from the organs in 

 other Mammals. Their copulatory organs consist of a short penis, 

 which is formed of two erectile bodies, and which lies in a pouch 

 which opens into the cloaca. By means of a muscle this can be 

 approximated to the urogenital canals, and so takes up the sperm 

 through an orifice which is placed at its root, near the opening of the 

 urogenital sinus. Owing to the special mode by which a portion of 

 the wall of the cloaca is differentiated, this organ comes to be 

 exclusively related to the generative apparatus, while the urine 

 passes out through the cloaca. 



When the cloacal aperture is differentiated into two orifices, the 

 copulatory organ becomes more closely related to the urogenital 

 sinus. During the embryonic stage a fold begins to be raised up 

 around the cloacal orifice, and a process is developed on the anterior 

 wall of the cloaca, which carries on its posterior surface a groove 

 which leads to the opening of the urogenital canal. As the embryo 

 continues to grow, the cloaca becomes shallower, and the wall of 

 partition between the orifice of the hind-gut and the urogenital 

 canal, which is formed from the lower end of the urachus, becomes 

 more distinct. At last the orifices which were formerly placed on 

 the floor of the cloaca come to the surface. The anterior fissure at 

 the base of the genital protuberance forms the opening of the uro- 

 genital sinus, while the hinder orifice forms the anus. In many 

 Mammalia the two orifices are always close to one another, and may 

 even be enclosed by the same fold of integument; in the female sex 

 the two orifices are ordinarily close together. This is most markedly 

 the case in the Marsupialia (where there is even a common sphincter 

 for the anus and urogenital orifice) and Rodentia ; in which forms, 

 indeed, it obtains in the males also. 



§ 458. 



The urogenital sinus is developed to a different extent in the 

 two sexes, and this is due to the difference in their functions. In 

 the male the urogenital sinus and genital protuberance grow out 

 into a narrower, but ordinarily long canal (the so-called urethra), 

 with the walls of which erectile organs are connected. They form 

 the penis. In the female there are parts which are similar to, 

 though less largely developed than, this organ and its erectile 

 bodies ; they form the clitoris, an organ which corresponds to the 

 penis. 



The erectile organs of the Marsupialia arc formed of two bodies 



