THE CORPUS LUTEUM OF PREGNANCY IN SWINE. 73 



ary name, corpus luteum, does not apply to swine, for the color of the cut surface of the 

 fresh organ is a light pinkish gray, which only changes to yellow in the old corpora. 



Besides the 1 to 10 corpora lutea in a given ovary, which experience shows mark the 

 site of the ovulation which led to the pregnancy, there are often a number of smaller cor- 

 pora lutea, which are bright yellow in color, and on section prove to consist mainly of dense 

 connective tissue. In some ovaries there is still another older generation, dating from the 

 second previous ovulation, which are seen as small white bodies nearly buried in the ovary. 

 The remains of corpora lutea older than the second previous generation are not usually 

 visible on gross examination, except as minute opaque bodies tucked in between the follicles 

 and appearing in sections as the familiar hyaline areas. Since the corpus luteum remains 

 as a distinct structure through the two succeeding ovulations, it follows that during the 

 sexually active period of life the ovary should never be free from corpora lutea. This was 

 true of the specimens examined by Kaeppeli (1908), but in our material there are many 

 which contain nothing but follicles and what are apparently corpora lutea several periods 

 old. Either these are atretic follicles, resembling corpora lutea, the sexually active period 

 not having begun, or else it is not uncommon for the sow to pass an cestrual period without 

 ovulation. 



If, then, the corpus luteum is still evident after two successive ovulations following 

 its formation, it becomes important to the progress of our study to know whether there is 

 any likelihood of confusing the corpora lutea oi two different generations. When two or 

 more generations are present, by microscopical study we always find such marked histo- 

 logical differences that there is no possibility of confusion, but on naked-eye examination 

 it is not uncommon to find corpora lutea apparently intermediate, in size and color, between 

 the two obviously different groups present. I have submitted all such cases, 8 in number, 

 to microscopical study, which showed that in 3 cases corpora lutea markedly smaller than 

 their mates presented the same histological appearances. A typical instance is given by 

 a specimen in which there were two corpora lutea of pregnancy 8 to 9 mm. in diameter, and 

 another of the same histological appearance only 4 mm. in diameter. In another case it 

 was necessary to section every corpus luteum of both ovaries to find how many of them 

 were recent. The moral is that critical points regarding the age of corpora lutea can not 

 be decided without microscopical evidence. 



Important also are the questions as to whether the corpora lutea of both ovaries, 

 dating from the same ovulation, present the same histological appearance, and whether all 

 the corpora of one ovary, dating from the same ovulation, are alike. To answer these 

 questions, I have examined sections taken from corpora lutea of both ovaries of 19 sows. 

 and a larger number taken from two or more corpora lutea of single ovaries, with the simple 

 result that all the corpora lutea of the same pregnancy, in both ovaries are in cytological 

 structure absolutely identical. Sometimes the central cavity of the follicle lingers longer 

 than in its mates, or there is a minor difference in size or other gross characteristic, but in 

 microscopic structure never. 



HISTOLOGY OF THE CORPUS LUTEUM OF THE SOW. 

 It is necessary, for the sake of clearness, to discuss briefly the perennial question of 

 the ovary — that of the origin of the lutein cells— which above almost all other anatomical 

 questions is notable for radical differences of opinion among the many capable investigators 

 who have attacked it. The reader will recall that there are three views as to the source 

 of the lutein cells of the mammalian corpus luteum. The first suggestion was that of von 



