CYCLES OF REPRODUCTION 7 



into the summer. Thus the yak in Russian Asia breeds in 

 July and August, has a gestation period of about 260 

 days, and a season of birth in the following spring. ^ 

 In the horse pregnancy lasts about 1 1 months and in the 

 ass about 12 months, so that breeding in one spring is 

 followed by the birth of tne young in the next.^ In such 

 animals the season of mating is only separated by a few 

 days from the season of birth. 



Mention must also be made of other factors that 

 prolong the time between mating and birth in some 

 mammals. In bats living and hibernating in temperate 

 regions there is a long interval between copulation and 

 fertilization. This curious state of affairs is well-known in 

 the British horseshoe bats of the genus Rhinolophus, ^^e 

 and in the little brown bat of North America. ^ Copula- 

 tion occurs during September and October, and the 

 females retain the spermatozoa in the vagina or uterus 

 until the eggs are produced in ApriL Pregnancy then 

 begins and the young are born in June or July. 



Another factor which has the same effect in causing a 

 wide separation between the times of breeding and, of 

 birth is the phenomenon of delayed implantation of the 

 embryo. This was first described by Bischoff, more than 

 a hundred years ago, in the case of the roe deer, which 

 has a breeding season in July and August.^ The fertilized 

 eggs develop for a few days to produce the blastocyst 

 stages, and these then lie dormant until December. 

 During that month they become implanted into the 

 uterine wall, and growth is rapid until the young are 

 born in May. The same phenomenon is common among 

 the Mustelidae (the badgers, stoats and weasels) in 

 which fertilization usually occurs in summer, the 

 blastocysts remained unimplanted until the following 

 January, and the young are born in March. i^* 



Similar delays appear to be rare among the lower 

 vertebrates, and no case has been found among the birds. 

 However, among reptiles mention may be made of the 

 curiously long incubation period of Sphenodon, the 



