MAMMALIAN SEXUAL CHARACTERS 145 



given off by the embryos in the marsupial pouch, 

 and these must be absorbed by the skin of the 

 pouch. In this way it seems to me we have a logical 

 explanation of the fact that the corpora lutea in the 

 Marsupial are not absorbed at parturition as in 

 Eutheria. As Sandes says the ' greater part of the 

 period of lactation,' it would appear that absorption 

 of the corpora lutea takes place when the young 

 Dasyurus have grown to some size, become covered 

 with hair, and are able to leave the teats or even the 

 pouch at will. Under these conditions it is obvious 

 that diffusion of chemical substances from the 

 young through the walls of the pouch would come to 

 an end. It would be interesting in this connexion to 

 know more of the relation of egg and embryo to the 

 pouch and to the corpora lutea in Echidna. In 

 Ornithorhynchus the eggs are hatched in a nest 

 and there is no pouch. 



On this view that the corpora lutea are the result, 

 not the cause, of intra-uterine gestation, it would no 

 longer be possible to maintain the theory that the 

 corpus luteum in the human species is the cause by 

 its internal secretion of the phenomenon of menstrua- 

 tion. This was the theory of Born and Frankel.^ 

 Biedl's conclusion is that the periodic development 

 and disintegration of the uterine mucous membrane 

 in the menstrual cycle is due to the hormone of the 

 interstitial cells of the ovary. Leopold and Ravana 

 found that ovulation as a rule coincides with 

 menstruation, but may take place at any time. 

 Here, again, the problem must be considered from 

 the point of view of evolution. It can scarcely be 

 doubted that the thickening and growth of the 



* See Biedl, Internal Secretory Organs (Eng. trans.). 101-, p. 404. 



K 



