G16 COMPARATIVE ANATOMY. - 



opens into the sinus urogenitalis. The uppei' portion^ which 

 commences by a very wide coelomic orifice, forms an oviduct 

 (Fig 351, od), while the next and thicker- walled portion forms a 

 uterus (m). Each of the two uteri open by a papilliform process into 

 a portion, which from the exterior appears to be common to them 

 both, and which is formed by the union of the two Mullerian 

 ducts. A curved vagina is given off from this on either side 

 (Didelphys), or the commencement of the tube is replaced by a 

 CEBcal vaginal sac which is pushed out backwards, and is usually, 

 though not always, divided internally by a median partition ; from 

 this sac the distinct " vaginal canals " (cv) pass in a curved direction 

 to the urogenital sinus (cug) (Halmaturus) . 



In the monodelphous Mammalia the archinephric ducts are united 

 with the Mullerian ducts to form a common chord (genital chord) . The 

 connection between the two Mullerian ducts, which is well marked in 

 Halmaturus, is effected in them at about the middle point of the duct, 

 and thus they become connected during embryonic life. A portion 

 of these ducts have their cavities fused, while they are separate in 

 front of, and behind this point ; this is an indication of the common 

 sac, which gives off the vaginal canals in the Marsupialia. But in 

 the Monodelphia the lumina are fused as far as the end of the genital 

 chord, and so form a single canal (genital canal) which opens into 

 the sinus urogenitalis. There are, therefore, two canals, which are 

 separated from one another at their commencement, but which unite 

 into an unpaired portion of varying length ; these canals are derived 

 from the Mullerian ducts, which are separate in the early stage of the 

 embryo. The parts, which are distinguishable even in the Marsu- 

 pialia, are due to the differences in the extent to which the walls of the 

 different parts are differentiated, and the modifications in them are 

 essentially due to the greater or less extent of the two tubes. The 

 uterus undergoes a number of changes, most of which are due to 

 adaptations to its relations to the foetus. Two completely separated 



uteri open into a 

 ^ ■" " vagina in many Ro- 



dentia (Lepus, Sciurus, 

 Hydrochoerus, etc.), 

 and in Orycteropus 

 (Fig. 352, A). In 

 other Rodentia the two 



Fig. 352. Various forms of the uterus A U C. n Uterus. uteri are Only united 

 od Oviduct. V Vagiua. for a short distance 



into a common open- 

 ing into the vagina (e.g. Cavia, Ccelogenys, Mus). This leads to the 

 arrangements seen in the uterus of the Insectivora, Carnivora, Cetacea, 

 and Ungulata, where a single uterus is continued into two separate 

 cornua (i?), which are continued into the oviducts. When the common 

 portion of the uterus is elongated, the cornua are shortened ; this is 

 the case in the Chiroptera and Prosimise ; in the Simias, as in Man, 

 there is a single uterus {G), which receives an oviduct on either side. 



