CYCLIC CHANGES IN THE OVARIES AND UTERUS OF THE SOW, ETC. 141 



THE UTERINE MUCOSA DURING THE EARLIEST WEEKS OF PREGNANCY. 



(Figures 31, 32, 33, plate 4.) 



It now becomes a matter of great interest to compare the histological state of 

 the uterus during the earliest weeks of pregnancy with the cyclic alterations just 

 described in the non-pregnant animal. The only available information on this 

 point is given in brief form by Assheton (1906) in his description of the ungulate 

 placenta. It has therefore seemed so important to make renewed observations on 

 this subject that a series of early pregnant uteri has been assembled (by the methods 

 outlined above, pages 123-124), including ova undergoing segmentation in the tubes 

 and uterus, morulse and early blastocysts, and implanting embryos in the shield and 

 earliest somite stages. These have been obtained from the butcher and are there- 

 fore without exact data as to age; they have been arranged into a series by com- 

 parison of the corpora lutea and the stage of the embryos. The probable ages in 

 days have been estimated from the data of Assheton (1898) on the rate of develop- 

 ment of the pig embryo and from our own studies of the development of the corpus 

 luteum (Corner, 1919). 



Examination of these very early pregnant uteri gives a result which is as im- 

 portant as it is simple. The same histological changes are found during the first 

 15 days of pregnancy as during the 15 days following an oestrus without copulation. 

 By the seventh day the cestrous epithelium has passed into the high columnar stage, 

 with hillocky arrangement of the surface; mitoses have ceased in the epithelium 

 but are numerous in the glands; eosiniphil leucocytes are present in large numbers 

 in the subepithelial stroma. Thus by the time the ova have developed into large 

 spherical blastocysts (eighth and ninth days) the uterus can in no wise be dis- 

 tinguished from a non-pregnant uterus 8 to 10 days after ovulation. (Compare 

 fig. 28 with fig. 31, pi. 4.) Two uteri containing wrinkled vesicles of age estimated 

 at 11 days show low columnar epithelium with smooth surface, diminution of the 

 hillocky arrangement of the epithelium, and few eosinophils. By the fourteenth or 

 fifteenth day, when the chorion is in contact with a large part of the mucosa, the 

 low columnar or cuboidal cells are covered with the curious frayed or rounded 

 protuberances which we have seen to be characteristic of the non-pregnant uterus 

 at the same time after ovulation (fig. 32, pi. 4). These changes of course affect 

 the more distant crevices and angles of the mucosa as well as those areas where the 

 chorion is applied to the uterine surface. Where the chorion has been separated 

 from its attachment to the epithelium, as inevitably occurs over large areas during 

 fixation, it may be seen that the trophoblast is pitted and roughened by contact 

 with the irregular surface of the epithelium (fig. 33, pi. 4). 



After this stage the identity of the histological processes in pregnant and non- 

 pregnant uteri is at an end. At a time when the non-pregnant uterus is again 

 undergoing precestrous changes (18 to 20 days) the pregnant uterus presents even 

 lower epithelial cells and greater roughening of the cellular surface, so that Assheton 

 speaks even of a degeneration of the epithelium at this time (fig. 33, pi. 4) . The 

 cells do not die off, however, but on the contrary again become of medium or high 

 columnar type, and so persist, as is well known, throughout pregnancy. The stage 



