THE CESTROUS CYCLE IN THE MAMMALIA 39 



and that in Echidna, as a general rule, only a single egg is 

 impregnated and developed at a time. After the egg is laid 

 (for Monotremes, as is well known, differ from all other 

 Mammals in being oviparous) the mother stows it away in her 

 pouch. This is always well developed at the sexual season, 

 after which it disappears, not to appear again until the ap- 

 proach of the next sexual season. Semon states that, although 

 the pouch is first visible in the embryo, it is thereafter lost to 

 sight until the beginning of the first prooestrum. 



Maesupialia 



It would appear probable that most Marsupials breed once 

 annually, but some are said to do so more frequently. Semon ^ 

 says that in the native Australian " bear " {Phascolarctus 

 cinereus), on the Burnett, the sexual season begins at the end 

 of October. Since he failed to find pregnant females until 

 the middle or end of November, it would seem that the 

 sexual season probably extends for three or four weeks. The 

 males at this time experience a rutting season, during which 

 they cry loudly, more frequently in the evening and night, but 

 also during the day. The gestation, as in all Marsupials, is 

 extremely short, the young being transferred at a very early 

 stage of development to the mother's pouch, as in the case of 

 Echidna.^ 



The kangaroos in the Zoological Society's Gardens in London 

 are stated to display sexual excitement in September, and also 

 in April. At such times a sHght flow of mucus, which may be 

 tinged with a little blood, has been observed passing from the 

 aperture of the vagina.^ It would appear, therefore, that 

 kangaroos may breed twice a year. Unfortunately, there is no 

 positive information available as to whether oestrus recurs 

 during the same sexual season. 



• Semon, he. cit. 



° In the bandicoot {Perameles) the young are nourished by an allantoic 

 placenta as in the higher Mammals. (See p. 384.) This is exceptional among 

 Marsupials. 



\ Wiltshire, " The Comparative Physiology of Menstruation," ^rit. Med, 

 Jour., 1883. 



