68 THE PHYSIOLOGY OF REPRODUCTION 



gestation period extends for thirteen months, the recurrence of the 

 sexual season is postponed by pregnancy for a whole year. Again, 

 in small animals like the rat, gestation only prevents the recurrence 

 of oestrus, reducing the number of dioestrous cycles, but not inter- 

 fering with the recurrence of the sexual season. " But whenever 

 gestation occurs it encroaches upon, if it does not entirely absorb, 

 the ancestrum ; that is to say, it reduces the period during which 

 the generative organs would lie fallow if the sexual season were a 

 barren one. Thus in the case of a mare, a barren sexual season may 

 consist of a series of dioestrous cycles lasting for as long as six 

 months, in which case the ancestrum lasts six months also, after 

 which another sexual season begins. A reproductive sexual season, 

 however, results in a period of eleven months' gestation, interfering 

 not only with the dioestrous cycles which would have recurred if 

 conception had not taken place, but also absorbing practically the 

 whole of the anoestrum." l 



The duration of the gestation period is intimately connected not 

 only with the size of the body, 2 but also with the stage of development 

 at which the young are born. 3 It is longest in the large terrestrial 

 and gigantic aquatic Mammals (Ungulata and Cetacea), \vhich live 

 amid favourable conditions of nourishment. With these animals the 

 young are so far advanced in development at the time of birth that 

 they are able to follow the mother about, and to a certain extent 

 shift for themselves. In Carnivores and Rodents the period of 

 pregnancy is relatively shorter, and the young are often born naked, 

 and with unopened eyes, and consequently are absolutely helpless 

 for a considerable time after birth. The gestation period is shortest 

 in the aplacental Mammals (Monotremata and Marsupialia), in which 

 the young are born at a very early stage and transferred to a pouch 

 formed by cutaneous folds in the vaginal region. In Monotremes 

 the young are hatched from eggs which, after being laid, are deposited 

 in the pouch. 



The question as to what are the precise factors which determine 

 the length of the gestation period has already been referred to in the 

 first chapter, where it was pointed out that both the duration of 

 pregnancy and the time of the year at which breeding occurs are 



1 Heape, loc. cit. 



2 The period of gestation is 144 days in Southdown sheep and 150 in 

 Merinos which are larger, while the hybrid is intermediate. (Lydekker, The 

 tiheep and its Cousins, London, 1912.) The causes which determine the variation 

 in the gestation period within any one species have been investigated by 

 Dussogno, who found that in pigs neither the size, weight, nor predominant 

 sex of the litter affected the gestation period, but that it appeared to vary 

 with the age, vigour, and condition of the sow. (" Sulla durata della gravidanza 

 nelle Scrofe Yorkshires," L'Industria latticed e Zootecnica, 1915.) 



3 Sedgwick, Student's Text-Book of Zoology, vol. ii., London, 1 905. 



