PARTURITION 201 



(see p. 169), that the induction of oestrus during pregnancy leads 

 to abortion or reabsorption. It is not improbable, therefore, 

 that the recurrence of the oestrus-producing stimulus, associated 

 with the atrophy of the corpora lutea at the end of gestation, is 

 connected with the mechanism of parturition. 



Prolongation of gestation by luteal stimulation. Since the 

 atrophy of the corpora lutea is a necessary prelude to parturition, 

 it would seem that the injection of active luteal extracts when 

 this atrophy is beginning should inhibit the onset of parturition. 

 Data on this point appear to be lacking, but a similar experiment, 

 i.e. the prolongation of luteal activity, has recently been carried 

 out by Teel on the rat (611). This author stimulated the luteal 

 tissue during pregnancy by the injection of NaOH extracts of 

 the anterior pituitary body (see p. 153 and p. 186), and ap- 

 parently, as a consequence of this, the period of gestation was 

 lengthened by two to six days. Teel's conclusions are as 

 follows: (a) the increase in the gestational period is due to a 

 delay in implantation of the embryos, (6) the foetuses eventually 

 die in ittero owing to the failure of the parturition mechanism, 

 and are expelled still-born, (c) this intra-uterine death is due to 

 severance of the placental attachment. The delay in the 

 implantation of the fertilized ovum is contrary to what would 

 be expected, but even if luteinization of the ovary during 

 pregnancy does not make possible a longer development of 

 the fcetuses, it clearly interrupts the normal mechanism of 

 parturition. 



[h) DIRECT ACTION OF THE OVARY ON SPONTANEOUS 

 UTERINE CONTRACTION 



In view of the clear connection between the ovarian cycle and 

 parturition, it might be supposed that some ovarian secretion 

 produced at a certain time in the cycle acted directly on the 

 uterus to cause the activity resulting in parturition. This 

 possibility is substantiated by the fact that the isolated uterus 

 shows in its spontaneous contractions a cyclic variation which 

 is correlated with the ovarian cycle. It has not been possible, 

 however, to show that ovarian extracts act on the uterus to a 

 degree necessary to produce parturition. C\C A / 



