GENITAL ORGANS 477 



(cf. p. 474). In many Lizards they are large and variously 

 coloured, and lie within the pelvic region ; in Snakes they extend 

 along almost the entire body-cavity. 



Among Birds, true hermaphroditism has been observed very 

 exceptionally in the Chaffinch. In some cases the ovary 

 may undergo structural changes, and no longer produce ova, the 

 female then taking on certain secondary sexual characters of the 

 male. 



Mammals. In Mammals the generative apparatus 110 longer 

 extends along the entire body-cavity, as in the lower Vertebrates, 

 but is confined to the lumbar and pelvic regions. Moreover, 

 in correspondence with the close relations which usually take 

 place between mother and embryo (p. 438), there is a much greater 

 differentiation of the generative organs than occurs in lower types. 

 The transition is not, however, a sudden one, for in the lowest 

 Mammals, viz., the Monotremes and Marsupials (Figs. 359 and 

 360), these organs show many points of resemblance to those of 

 Reptiles and Birds. 



In the oviparous Monotremes 1 the left ovary is better 

 developed than the right, and each has the appearance of a 

 bunch of grapes : the cloaca persists, and the oviducts (Miillerian 

 ducts), which in other Mammals become more or less fused with one 

 another proximally, remain distinct throughout, and open into the 

 urinogenital canal anteriorly to the ureters and bladder. 



In the higher Mammals the oviducts become distinctly differ- 

 entiated into three portions, a Fallopian tube, a uterus, and a 

 vagina. The vagina opens to the exterior (Figs. 339, 360, and 361), 

 while the Fallopian tube communicates with the abdominal cavity 

 by a funnel-shaped aperture which is usually fimbriated and 

 ciliated. 



In Marsupials the ovaries vary much in form, and the fusion of 

 the two oviducts is much less marked than in the higher Mammals : 

 in order to trace the gradual differentiation, of these parts, their 

 condition in Opossums (Didelphidse) will first be considered. 



A dilated portion of each oviduct (Fig. 360, A), giving rise to a 

 uterus, is plainly distinguishable from the rest, and its narrowed 

 posterior end comes into close contact with its fellow in the middle 

 line. At this point (f ) each uterus communicates with the vagina 

 by a distinct os uteri. The vagina curves sharply outwards, and, 

 then backwards, opening close to its fellow into the elongated urino- 

 genital canal. The ureters, as in all other Marsupials in which the 

 vaginre have a similar arrangement, pass between the curved 

 portions of the vaginae to the bladder. 



From the condition of the female generative organs in 



The eggs (usually one in Echidna and two in Ornithorhynchus), surrounded 

 by a thin shell of keratin, have only been found in the left oviduct, in which 

 their early development takes place. 



