DEPARTMENT OF EMBRYOLOGY. 



91 



the cells take place, by which the epithelium assumes the state charac- 

 teristic of pregnancy at the implantation stage. If no embryos are 

 present the same changes occur, but they subside after the fifteenth 

 day, the uterine mucosa undergoing a slow reversion to the oestrous 

 tj^pe. These histological changes undoubtedly have a functional 

 value which readily suggests itself, but which remains to be established. 

 It is clear, however, that in the sow the uterine cycle includes an 

 upbuilding of the mucosa, presumably under control of the corpora 

 lutea, which is favorable to 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 em- 



PREGNANCV 



Fig. 1. — Diagram prepared by Dr. Comer showing the cyclic changes in the ovary of the 



pregnant and non-pregnant sow. 



bryos) subside, as do the corpora lutea, in preparation for a new 

 ovulation. 



Working on an allied investigation. Dr. Corner has found clear 

 evidence that the ovum of the pig, in passing from the ovary to the 

 site of implantation, very frequently migrates through the uterine 

 horn of its own side and the connecting body of the uterus to the 

 opposite horn, where it becomes definitely implanted. This study 

 was made possible through the cooperation of the Station for Experi- 

 mental Evolution, where facilities were provided for the examination 

 of a large number of uteri and ovaries from recently slaughtered sows. 

 If one records the number of corpora lutea in a large series of pregnant 

 animals, it will be found that very commonly there are more in one 



