576 THE PHYSIOLOGY OF REPRODUCTION 



degenerative change] in the. placenta at term must be regarded as 

 evidence of its senility, and that this change is analogous to the 

 obliteration and atrophy of the chorion Iteve at an earlier period. 

 Where these changes are marked the nutrition of the foetus must 

 be interfered with, and it is possible that certain of its metabolic 

 products may result _ in stimulation of the uterine centre." ^ This 

 theory should be compared with that advanced by Spiegelberg (see 

 above). 



(11) Lastly, Williams calls attention to the fact that excessive 

 physical fatigue, sudden jars or violence, mental emotion, fright, etc., 

 may lead to the termination of gestation in women. Similarly, it is 

 well known that circumstances of a like nature may induce abortion 

 in animals. 



Wilhams remarks that in all probability the onset of labour in 

 most cases is due to a combination of a number of the above- 

 mentioned causes. The main objection to all the theories which 

 have so far been advanced is that they take no account of the 

 complexity of the problem. An hypothesis may be fairly adequate 

 as a general explanation of the duration of gestation, while at the 

 same time taking no account of the immediate cause of birth. Thus, 

 it is no doubt true that the time of parturition is determined largely 

 by the necessities of the offspring, but this conclusion provides iio sort 

 of explanation as to why it is that the process in any one particular 

 species generally commences at a certain fixed stage of development, 

 and it remains for us to assume that it is one of the inherent 

 properties of the uterus and correlated organs in the species in 

 question that it should do so. Even- on this assumption it is 

 impossible to avoid concluding that there must be some definite 

 exciting cause. 



Some light is thrown on this question by the study of the 

 condition of pseudo-pregnancy (see pp. 33 and 36). The three 

 animals in which it is known are the dog, the marsupial cat, and 

 the rabbit (the latter only under experimental conditions). In each 

 of these species during pseudo-pregnancy there is a persistent corpus 

 luteum which at or near the end of the period enters into a state of 

 involution and so probably is in a condition which is not dissimilar 

 to that of the corpus luteum verum at the end of gestation. Further- 

 more all these animals at the termination of pseudo-pregnancy ^ display 

 habits or instincts which are visually associated wi^h the act of 

 giving birth. Thus the bitch may prepare a bed as if for a litter 

 of pups, the female Basyurus cleans out her pouchi as if for the 

 reception of young, and the doe rabbit plucks her breast of fur and 

 uses it to line a nest. Since these habits are displayed at the end 



1 Williams, loc. cit. ^ In the rabbit about the eighteenth day. 



