CYCLIC CHANGES IN THE OVARIES AND UTERUS OF THE SOW, ETC. 143 



IV. SUMMARY AND DISCUSSION. 



It remains to consider how far our results have justified the hope, with which we 

 began, that the reproductive cycle of the sow might prove illuminating in propor- 

 tion to its simplicity. We have found, first, that there is a regular periodic chain 

 of events in the ovary; beginning with rupture of the follicle during oestrus, the 

 corpora lutea attain complete organization about the seventh day and hold their 

 full development until the fourteenth or fifteenth day. We have seen that this 

 interval is just long enough to cover the period during which the embryos are 

 becoming attached, should pregnancy result from the ovulation. If no pregnancy 

 occurs, atrophy of the corpora lutea begins about the fifteenth day. 



In the uterus we have seen a correlated cyclic alternation. During oestrus 

 there is a characteristic state of the uterine mucosa very similar to that which has 

 been described by Loeb (1914) and by Stockard and Papanicolaou (1917) in the 

 guinea-pig, and very fully and accurately by Long and Evans in their forthcoming 

 monograph on the rat. During the time of formation of the corpora lutea the 

 epithelium, glands, and stroma undergo a series of changes which attain their 

 height about the same time that the corpora become solid. From the eighth to 

 the tenth day the uterine epithelium presents the appearance of active serous secre- 

 tion, and then its cells lose height, subside to a low columnar or cuboidal form, and 

 become marked by cytoplasmic protrusions or roughenings on their free ends. After 

 the fifteenth day, when the corpora lutea begin retrogression, there is a slow rever- 

 sion to the cestrous type of structure. A key to the understanding of these changes 

 is found in the fact that an exactly similar histological progression occurs during 

 the first two weeks of pregnancy, no doubt serving as a mechanism to permit 

 attachment of the embryos. Recalling the elementary type of implantation in this 

 species, one is tempted to form the simple, perhaps crude hypothesis that the 

 uterine epithelium by its serous-secretory stage provides for flotation of the delicate 

 embryonic vesicles and thus facilitates their migration and spacing in the relatively 

 extensive uterine cavities (see Corner, 1921); but during the period of attachment 

 the mucosa is, on the other hand, rendered glutinous by the peculiar surface rough- 

 ening of the epithelial cells, in order to assist in attachment of the chorions. 



Whatever the functional value of these histological details, the reader will 

 agree that the processes described in this paper strongly suggest that the underlying 

 fact of the uterine cycle of the sow is an upbuilding of the mucosa, presumably 

 under control of the corpora lutea, for the purpose of successful implantation. 

 Each act of ovulation is thus accompanied and followed by uterine changes, which 

 either go on to placenta formation or (in the absence of embryos) subside once more, 

 as do the corpora lutea, in preparation for a new ovulation. 



Such a general conception has long since been suggested by the work of a num- 

 ber of investigators, including that of Hitschmann and Adler (1908) on the human 

 uterus, L. Loeb (19116, 1914) on the guinea-pig, Keller (1909) on the dog, Ancel 

 and Bouin (1910) on the rabbit, and Hill and O'Donoghue (1914) on Dasyurus 

 viverrinus. These writers did not see the changes exactly as we have seen them in 



