ORIGIN OF THE CORPUS LUTEUM 135 



follicles and corpora lutea of 171 sows of which there are records 

 sufficient to determine the stage of the reproductive cycle. In 

 162 of them the ova, developing embryos, or foetuses were 

 examined and recorded. 



Twenty-four were killed during the oestral period or within 

 the first week after the onset of heat. Some of the tubal ova 

 found were unfertilized, others were fertilized and were in stages 

 from the one-celled to the six-celled embryo. Five of the twenty- 

 four sows mentioned were obtained before a method of discover- 

 ing the ova had been acquired, and the ova were therefore not 

 sought, but as the dates of copulation were noted at the Univer- 

 sity of California Farm by Professor Thompson, it seems proper 

 to include them, since their corpora lutea agree with the others 

 in structure. 



Six sows were taken in the second week after ovulation. As 

 the ova were unfertilized, they had degenerated, and were not 

 found, except shriveled eggs in two of the sows. It chanced that 

 none of those sows which had copulated were killed during this 

 period, and thus the opportunity to obtain embryos of the second 

 week did not fall to my lot. 



Fifteen contained embryos of the third week, from five to 

 thirty-nine somites. The ovaries of the eleven youngest of these 

 were given me by Prof. F. R. Sabin; some of the embryos to 

 which they were related are described and pictured in her recent 

 contribution to the early vasculogenesis of the pig (Carnegie 

 Institution of Washington, Contributions to Embryology, No. 

 18, 1917). 



One hundred and twenty-four compose a complete series from 

 animals containing embryos of the fourth week to the end of 

 pregnancy, the embryos or foetuses being measured in each case. 

 Two were obtained from sows which had littered seven and ten 

 days before killing, respectively. Most of the older corpora 

 lutea of pregnancy were prepared in the Anatomical Laboratory 

 of Johns Hopkins University, and formed part of the material for 

 my previous monograph ('15). They have been restudied in the 

 light of the results gained from the specimens of the first fourteen 

 days after ovulation, all of which were obtained in California. 



