560 



MARSUPIALS 



XX. I 



pseudo-vagina or birth canal. This may then close until the next 

 parturition. As in monotremes the rectum and urinogenital sinus open 

 together at a common cloaca, though this is not very long, longer in 

 the female than the male (Fig. 337). There is a well-developed penis, 

 often bifid at the tip, in which case the clitoris is also double, the 



Fig. 336. Kangaroo, female genitalia. Note that the sinus vaginalis opens 

 directly into the urogenital sinus. Labelling as for Fig. 337. (After Brass and 



Ottow.) 



arrangement presumably ensuring fertilization of both oviducal tubes. 

 The testes descend to a scrotum. 



The reproduction of marsupials shows a viviparous condition that 

 is not closely similar to any found in placentals. The egg is rather 

 yolky and covered with albumen and a membrane; cleavage is very 

 unequal. In some forms (Dasyurus) there is a contact of the vascular 

 wall of the yolk-sac with the somewhat hypertrophied uterine wall 

 (omphaloidean placenta). Only in the bandicoot, Perameles, does 

 the allantois develop a nutritive function to some extent. In most mar- 

 supials no placental arrangement develops at all, instead, uterine milk 

 may be taken up by the yolk-sac. The embryos are born very young, 

 as little as 8 days from conception in the opossum (Fig. 335). They 

 crawl along a track of saliva that is laid between the cloaca and the 



