34 INTERNAL SECRETIONS OF THE OVARY 



epithelium and glands. At the end of pseudo-pregnant 

 degeneration, anoestrous quiescence supervenes. 



In the early stages of pregnancy, changes similar to those of 

 pseudo-pregnancy are found. Subsequently, of course, the 

 development of both uterus and mammary glands is in excess 

 of that found in the non-pregnant animal. Lactation occurs 

 in what would be the next anoestrous period, so the compli- 

 cation of coincident oestrus and lactation does not appear. 

 For the same reason the bitch, unlike rodents, cannot suckle 

 and gestate at the same time. 



{c) GUINEA-PIG AND COW 



The guinea-pig and cow both have a dioestrous cycle contain- 

 ing a luteal phase, which has been shown experimentally to be 

 under the control of the corpus luteum. They are also similar in 

 having a relatively long period of gestation, so that the young 

 of both are born in an advanced stage of development. 



Guinea-pig. Early workers on the oestrous cycle in the guinea- 

 pig include Bischoff (67), Reichert (528), Hensen (289), Rein 

 (532), and Rubaschkin (539). These workers all recognised that 

 'heal' occurred very soon after parturition and that ovulation 

 was spontaneous. They failed, however, as did early workers on 

 the rat and mouse, to trace the cycle without taking parturition 

 as the starting-point. During the last twenty years very 

 extensive investigations on the uterine, ovarian, and mammary 

 cycles in the guinea-pig have been carried out by Loeb (379, 

 389, 391, 393), while more recently the whole subject has been 

 reinvestigated by Stockard and Papanicolaou (599), who 

 elucidated the vaginal cycle and introduced the vaginal smear 

 technique. According to Stockard and Papanicolaou the length 

 of the dioestrous cycle is about sixteen days (or rather longer in 

 winter); Loeb's observations agree roughly with this figure, 

 except that he found greater variability and a tendency towards 

 long cycles after copulation. Voss (625) gives the length of the 

 whole cycle as varying between thirteen and twenty days, of 

 which dioestrus occupies nine to sixteen days. 



Sterile copulation exerts no such influence on the time of 

 appearance of the next oestrous period as it does in the rat 



