320 COMPARATIVE ANATOMY. 



Only slight remnants of the mesonephros and Wolffian duct remain in the 

 female in Reptiles, and these undergo fatty degeneration. They lie asym- 

 metrically, arranged in a single row on either side, between the oviduct "and 

 vertebral column. The remains of the Wolffian duct are more marked in 

 female Snakes, Chelonians, and in Geckos than in other Lizards. 



The testes of Sauropsida correspond in position with the 

 ovaries, and, like them, increase in size in the breeding-season. 

 They have an oval, round, or pyriform shape (Figs. 242 and 250, 

 Ho), and are made up of greatly convoluted seminal tubules, held 

 together by fibrous tissue. In Reptiles (Lacerta, Anguis), " yellow 

 bodies," which correspond to suprarenals, lie along the outer 

 border of the testes, and at this point transverse canals pass out 

 from the testis to the parorchis (Figs. 242 and 250, Ep). 



The latter consists of greatly convoluted canals, and from 

 it arises the vas deferens (Wolffian duct), which either takes a 

 straight course, or is more or less coiled (Figs. 242 and 250, Vd}. 

 In Birds it opens by an independent aperture (Fig. 242, Vd l ) into 

 the cloaca, while in Lizards it fuses with the ureter shortly before 

 entering the latter. 



Remains of the anterior portions of the Miillerian ducts are present in the 

 male, their position corresponding with those of the female. Their lumen is 

 not continuous throughout, but the abdominal aperture may remain open 

 (Emys europaea). 



Lymphoid organs are present in many Reptiles, and probably have a 

 physiological relation to the generative organs (comp. p. 317). 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. 



Mammals. In Mammals the generative apparatus no longer 

 extends along the entire body-cavity, as in the lower groups of 

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

 Moreover, in correspondence with the close relations which take 

 place between mother and embryo, there is a much greater differ- 

 entiation 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 1 and Marsupials, these organs show many 

 points of resemblance with those of Reptiles and Birds (comp. 

 Figs. 251 and 252). 



Thus in Monotremes the left ovary is more strongly 

 developed than the right, and each has the appear- 

 ance of a bunch of grapes; the cloaca persists, and the 

 Miillerian ducts (oviducts) remain distinct from one 

 another. A more or less complete separation of the oviducts is 

 also seen in Marsupials, and this point deserves special attention 

 on account of its important morphological significance. 



In order to explain the gradual differentiation of these parts, 

 their condition in the Didelphida3 (Opossums), which come 



1 It has been recently proved that both Ornithorhynchus and Echidna lay eggs 

 (Caldwell and Haacke) (comp. p. 5). Further details on these points are to be 

 expected shortly. 



