156 THE PHYSIOLOGY OF REPRODUCTION 



much doubt that the changes taking place are identical with those 

 seen in the young ovary, which lead to ovogenesis, and therefore it 

 would appear that ovogenesis also takes place in the adult animal 

 during pregnancy." l 



Thus it would seem that the interstitial cells, which, like the ova, 

 are almost certainly derived from the germinal epithelium, are 

 actually potential ova, being capable of developing into true ova 

 when the appropriate stimulus is given. This stimulus is provided 

 by pregnancy, at which period they undergo enlargement so as to 

 exceed the size of a primordial ovum, and in addition pass through 

 the same series of nuclear transformations as those which characterise 

 embryonic oogenesis. 2 



THE SIGNIFICANCE OF THE PROCESTROUS CHANGES 



Having discussed the conditions under which the Graafian follicles 

 ripen and discharge in various species of the class Mammalia, we are 

 now in a position to consider more fully the significance of the uterine 

 changes with which ovulation is frequently associated. 



Many obstetricians have adopted the view that the degeneration 

 stage of menstruation in the human female is of the nature of an 

 undoing of a preparation (represented by the 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, 3 and was subsequently .accepted by His. 4 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 Mammalia, 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. 



It is possible, however, that in man the breaking-down stage 

 represents pseudo- pregnant degeneration as well as prooestrous 

 destruction owing to the two processes having become telescoped into 

 one another as a consequence of the shortening of the cycle, while, as 

 Hammond points out, the uterine congestion in the rabbit is greatest 

 at the end of pseudo-pregnancy and just before the onset of a new 

 ccstrous period. In the monoestrous dog, however, prooestrous 

 destruction and pseudo-pregnant degeneration are distinct. 



1 Lane-Claypon, lor. fit. 



- Cesa-Bianchi (loc. cit.) comments on the close resemblance between luteal 

 and interstitial cells. 



3 Sigismund, " Ideen iiber das Wesen der Menstruation," Berliner Klin. 

 Wochenschr., 1871. 



4 His, Anatomic Menschlicher Embrt/onen, 1880. 



