162 THE PHYSIOLOGY OF REPRODUCTION 



previous growth stage) for an ovum which failed to become 

 fertilised (or even to be released from the ovary). This theory 

 was originally put forward by Sigismund,^ and was subsequently 

 accepted by His.^ It has been summarised in the well-known 

 dictum that " women menstruate because they do not conceive." 

 It has been shown above, however, that menstruation in the 

 Primates is the physiological homologue of the prooestrum in the 

 lower MammaUa, and that ovulation in the latter occurs usually, 

 so far as is known, during oestrus, or at any rate not until after 

 the commencement of the destruction stage of the prooestrum. 

 Consequently Sigismund's theory becomes untenable. 



Loe wen thai ^ advanced the somewhat similar theory that the 

 monthly bleeding is actually brought about by the death of the 

 ovum in the uterus, the " decidua " of menstruation being pro- 

 duced by the embedding therein of the unfertilised egg. No 

 evidence has been adduced in support of this view, which is 

 evidently open to the same objection as Sigismund's hypothesis. 



A further modification of the same theory has been ad- 

 vanced by Beard,* who expresses the behef that the process of 

 menstruation is of the nature of an " abortion of something 

 prepared for an egg given off at or after the close of the pre- 

 ceding menstruation, and [that] it takes place because this egg 

 has escaped fertilisation." " Prior to the appearance of the 

 menses the uterus has formed a decidua, which is regarded as 

 equivalent to that which would arise when a fertilised egg be- 

 came affixed to the uterus." This theory also, if it is to be 

 entertained at all, necessitates the assumption that there is no 

 correspondence between the prooestrum in the lower Mammalia 

 and menstruation in the Primates, since the degeneration stage 

 of the prooestrum in the dog or ferret, for instance, can hardly 

 be of the nature of an abortion of something prepared for an 

 ovum which was discharged at the preceding " heat period " 

 many months before. The difficulty is further increased for 

 those animals which experience oestrus only once a year, or even 



1 Sigismund, " Ideen iiber das Weseu der Menstruation," Berliner Klin. 

 Wochenschr., 1871. 



^ His, Anatomie MensMicker Emhryonen, 1880. 



' Loewenthal, " Eine neue Deutung des Menstruationsprocesses," Arch. f. 

 GynaJc., vol. xxiv., 188i. 



* Beard, The Span of Gestation and the Cause of Birth, Jena, 1897. 



