302 



CHORDATE ANATOMY 



united Miillerian ducts. The duplex character of the vagina appears 

 in the marsupials (didelphians) which have two vaginae. In placental 

 mammals, however, the vagina is single. Four stages in the increasing 

 fusion of the uteri are represented in the placental mammals — uterus 

 duplex, bipartitus, bicornis, and, in primates, uterus simplex. Even 

 in the primates, however, the original duplex character of the Miillerian 

 ducts is retained in the paired uterine tubes. (Fig. 269) 



Oviduc-t 

 -" Uterus '"' 



--Vagina 



D. 



Fig. 269. — Four types of uteri occurring in different groups of mammals. A, 

 duplex, the type found in rodents; B, bipartite, the type found in certain carnivores; 

 C, bicornate, the type found in most insectivores and prosimians; D, simplex, the type 

 characteristic of the primates. (From Patten's "Embryology of the Pig," after 

 Wiedersheim.) 



External genital organs also make their appearance in mammals. 

 Copulatory organs are, however, not wholly new in this group. Some 

 of the flatworms have an intromittent organ by which sperm is conveyed 

 to the seminal receptacle of another individual. In elasmobranchs, 

 the pelvic fins of the male are modified as claspers which in copulation are 

 inserted into the cloaca of the female. None of these structures, however, 

 are morphologically comparable with the phallus of mammals. 



The free posterior extremity of the paired erectile folds (seminal ridges) 

 in the floor of the cloaca of Chelonia and Crocodilia forms a glans. In 

 monotremes likewise, the phallus points posteriorly and is contained 

 in the floor of the cloaca. In the walls of the monotreme cloaca, the 



