436 Mr. Hammond and Dr. Marshall. Correlation betiveen 



of the corpora lutea are essentially similar to those described for Dasyurus 

 in the period of pseudo-pregnancy. The close likeness between the sections 

 of the rabbit's uterus and the figures given in Hill and O'Donoghue's paper 

 has been commented on above. Moreover, the processes which take place in 

 the ovaries and mammary glands are also clearly of an identical nature in 

 the two animals. We suggest, therefore, that the uterine changes which go 

 on in the pseudo-pregnant uterus in tbe marsupial are not comparable to the 

 pro-cestrous changes of the Eutherian, as Hill and O'Donoghue suppose, but 

 are identical with those in the pseudo-pregnant rabbit's uterus, both being 

 dependent upon the formation of corpoi'a lutea in the ovaries. It is possible, 

 however, that the uterine congestion occurring near the close of the pseudo- 

 pregnant period is of the nature of a pro-cestrous congestion, since pseudo- 

 pregnancy (like true pregnancy) would probably in some cases have been 

 followed by another cestrous period, had the animals been permitted to live. 



It has been shown by Hill and O'Donoghue that in the marsupial cat there 

 is only one sort of corpus luteum, the duration of which is presumably always 

 the same. In the rabbit, also, there is only one kind of corpus luteum occurring 

 in correlation with either pregnancy or a condition comparable to pseudo- 

 pregnancy. The existence of only one kind of corpus luteum (which lasted 

 for an identical period, irrespectively of whether or not ovulation was suc- 

 ceeded by pregnancy) was no doubt the condition common to all primitive 

 mammals, and it seems probable that the shortening of the duration of the 

 " periodic corpus luteum "* was associated with the development of the 

 polycestrous habit from a state of moncestrum. For it is known that 

 ovulation cannot ordinarily occur in the presence of fully developed corpora 

 lutea, which, if they persist, cause follicular atrophy and inhibit the develop- 

 ment of ripe ova. Consequently it would be disadvantageous for such animals 

 to have periodic corpora lutea persisting for as long a period as corpora lutea 

 associated with pregnancy. 



In moncestrous animals, such as the dog, the persistence of the corpus luteum 

 over a period equivalent to pregnancy would not be detrimental to fecundity, 

 while we have shown above that there is evidence (derived from numerous 

 cases where bitches have been known to secrete milk nine weeks after 

 oestrus) that even in the dog such a persistence may occur. Moreover, the 

 great variability which different individual dogs experience in the recurrence 

 of oestrus is suggestive of a variation in the period over which the corpus 

 luteum persists. It may be that in moncestrous animals the primitive con- 

 dition occurring in Dasyurus, in which there is one sort of corpus luteum 

 only, continues to exist or is reverted to in certain individuals. 



* Or corpus luteum spuriuru. 



