428 Mr. Hammond and Dr. Marshall. Correlation between 



The interval between two cestrous periods (that is the interval occupied 

 either partly or wholly by pseudo-pregnancy) was from 22 to 30 days, the 

 period of gestation in the normal rabbit being 30 days. 



Whether or not the corpus luteum plays any part in mammary growth- or 

 secretion in the latter stages of normal pregnancy is a point which has not been 

 determined. 



In normal pregnancy the development of the glands is undoubtedly greater 

 than anything that occurs in pseudo-pregnancy, and it would seem probable 

 that some further factor is concerned in bringing about this growth. This 

 factor is possibly to be sought for either in the placenta, as suggested by 

 Basch, or in the myometrial gland, as supposed by Ancel and Bouin. Never- 

 theless, it is clear that the presence of corpora lutea alone, apart from the 

 existence of any subsidiary factor, suffices to stimulate gland growth to such 

 a degree of completion as to result in the secretion of milk. 



As mentioned already, Ancel and Bouin distinguish between the corpora 

 lutea of pregnancy and the so-called " periodic corpora lutea " which only occur 

 in animals that ovulate spontaneously. The artificially produced corpora lutea 

 in the rabbit are regarded as belonging to the former kind. Moreover in those 

 animals (like the rabbit) which only ovulate after coition the interstitial cells 

 are supposed to take the place of the periodic corpora lutea. It may be doubted 

 whether the distinction made between the two kinds of corpora lutea by Ancel 

 and Bouin should be insisted upon. In the first place the corpora lutea are all 

 formed in precisely the same way from the discharged follicles, while according 

 to Biedl the ovarian interstitial cells in rodents arise from connective tissue 

 which grows inwards so as to fill up the cavities of degenerate follicles. Such 

 cells are designated by Seitz " theca lutein cells " since they arise in the 

 theca interna of the follicles, and subsequently develop into cells resembling 

 those of corpora lutea. Miss Lane-Claypon, however, states that the ovarian 

 interstitial cells are derived, like the follicular epithelial cells, from the 

 germinal epithelium.* 



Furthermore, from the account given by Hill and O'Donoghue it would 

 seem that the corpora lutea in Dasyurus always undergo the same degree of 

 development irrespectively of the occurrence of pregnancy. They describe an 

 animal as being seen to clean out its pouch for the reception of young, although 

 it had not become pregnant, thus showing that in Dasyurus the cyclical changes 

 of the sexual organs, which are apparently consequent upon ovarian changes, 

 may even extend to the instincts associated with parturition and the nursing 

 of the young, although pregnancy had not taken place. 



* I have noted the presence of interstitial cells in the ovary of the rabbit prior to 

 the maturation of any follicles. — J. H. 



