144 MAMMALIAN SEXUAL CHARACTERS 



periods,^ and the correlation between the retained 

 developing ova and the hypertrophy of the ruptured 

 foUicles is comprehensible on my theory of the 

 influence of substances absorbed by the walls of 

 oviduct or ovary from the developing ovum. 



The case of Dasyurus, however, seems inconsistent 

 with this argument, for, as previously mentioned, 

 Sandes found that in this Marsupial the corpora 

 lutea persisted during the greater part of the period 

 of lactation, which continues for four months after 

 parturition. During the whole of this time there 

 are no embryos in the uteri, and therefore it might 

 be urged absorption of hormones from the embryos 

 cannot be the cause of the persistence of corpora 

 lutea in pregnancy. But it seems to me that a 

 complete answer to this objection is supplied by the 

 peculiar relations of the embryos to the pouch in 

 Dasyurus and other Marsupials. The skin of the 

 pouch while the embryos are in it is very soft, con- 

 gested, and glandular ; at the same time the embryos 

 when transferred to the pouch at parturition are 

 very small, immature, and have a soft delicate skin. 

 The relation of embryos to pouch in Dasyurus, there- 

 fore, is closely similar to that of embryos to uterus 

 after the first few days of pregnancy in the Eutheria. 

 It is true there is no placenta, but the mouths of the 

 embryos are in very close contact with the teats, and 

 both the skin of the embryos and that of the pouch 

 are soft and moist. If any special substances are 

 given off by the embryos in the uterus in ordinary 

 gestation, the same substances would continue to be 



^ According to Geddes and Thomson {Evolution of Sex, 1889), the 

 common grass-snake has been induced under artificial conditions to 

 bring forth its young alive. 



