xxv] 



METATHERIA 



659 



rapidly. The allantois, however, always remains small and though 

 in one or two cases it may contract an adhesion with the outermost 

 layer of cells of the developing egg and through this with the wall 

 of the womb yet this adhesion is temporary and does not persist till 

 birth. Most of the nourishment is absorbed through an adhesion 

 between egg and womb in the ventral region of the yolk-sac. Be- 

 neath the uterus comes the lowest part of the egg-tube, the so-called 

 vagina. The two vaginae come into close contact with each other 

 above and then diverge, both opening below apparently into the 

 lowest part of the bladder, as do the vasa deferentia in the male. 

 What seems to be the lowest part of the bladder is really the front 

 portion of the cloaca, which has become separated from the part 

 behind that receives the opening of the intestine. This common 

 vestibule for excretory and reproductive ducts is called the urino- 

 genital sinus, and its opening is distinct from that of the intestine 

 or anus, although the two openings are still surrounded by a common 

 muscle. From the spot where the vaginae meet above a pouch 

 called the median vagina is often developed. This ends blindly 

 in the young female, but in the mature female it acquires an 

 opening into the urino-genital sinus and through this opening the 

 embryo is born, the lateral vaginae serving merely to admit the 

 spermatozoa from the male. 



The penis or copulatory organ is a 

 thickening surrounding the opening of the 

 urino-genital sinus in the male, it is 

 directly continuous with the neck of the 

 bladder, and through it with the vasa 

 deferentia. 



When the young are born they appear 

 not as eggs but as little mammals, which 

 are however exceedingly small in size. 

 They are then placed by the mother, who 

 is said to transfer them with her lips, in 

 a pouch made by a fold of skin on the 

 lower part of her body, whence the name 

 Marsupials (Lat. marsupium, a pouch), 

 often given to these animals. A pair of 

 sesamoid or epipubic bones run forward 

 from the pubes. They are ossifications 

 of a tendon of the external oblique 

 muscle. Similar structures are found in 



FIG. 328. Diagram to illus- 

 trate the arrangement in 

 the female genital ducts of 

 the Metatheria. 



1. Ovary. 2. Oviducal 

 funnel. 5. Fallopian 

 tube. 6. Uterus. 



7. Vagina. 8. Median 

 vaginal pouch. 9. Uri- 

 no-genital vestibule. 



422 



