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 u'strus, reducing the number of dio-strous 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 ana-strum ; 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 dicestrous cycles lasting for as long as six 

 months, in which case the aim-strum 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 di<rstrous cycles which would have recurred if 

 conception had not taken place, but also absorbing practically t In- 

 whole of the amestrum." 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.-' 5 It is longest in the large terrestrial 

 and gigantic aquatic Mammals (Ungulata and Cetacea), which 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 liodents 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 shorte>t 

 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 i|uestion 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 occur* are 



1 Heape. In,-, .-it. 



- Tlif period of gestation i.s 111 days in Southdown slice)) and |.">o in 

 Merinos which arc larger, while the hyhrid is intermediate. (Lydekkcr, Tl- 

 >'/,<<// mill i'/ < 'tiit.o'nx. London, I!M) The causes which determine the variation 

 in the gestation period within any one species ha\c liccn investigated liy 

 I Missogno, who found that in pigs neither the si/.-, \\eight, nor predominant 

 >e\ of the litter aH'erted the gestation period, Imt that it appeared to vary 

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



liclle Scrofe Yorkshire-." /////"*/>/<> /r////w .' /nnti'rni,-ii. I'.H.'i. 



; Scdgwick, !<t'i'l>-itif T>''t-r>*>ok <>i Zonlo'j;/. vol. ii., London. l!>o.~>. 



