82 



THE CORPUS LUTEUM OF PREGNANCY IN SWINE. 



which the material was studied. As stated in the introduction, all the specimens had been 

 prepared in the same way. All the specimens from pregnancies of 20 to 25 mm. were taken 

 together and carefully studied, then all those from pregnancies of 50 to 55 mm., and so forth; 

 and in this way was obtained an idea of the nature of the corpus luteum at a series of stages 

 from an early period to term. Next the specimens of intervening length were examined, and 

 finally from these observations it was found possible to divide the life of the corpus luteum 

 into a number of definite periods according to the length of the fetus. When I was sure 

 of being on firm ground, I began to go through box after box of sections, registering a diag- 

 nosis of the age of each corpus luteum, and then looking it up in the records. With increas- 

 ing experience it became possible to place the specimen in its proper period practically every 

 time, never varying from the correct stage by more than one of the seven periods of preg- 

 nancy. Of course these periods into which I have found it possible to rank my specimens 

 are purely empirical, and represent lines which I have drawn at the limits of my ability to 

 distinguish the stages of the cytological changes. I had at first several more periods inter- 

 polated between those given, but found that I could not regularly distinguish them. In 

 short, it can generally be told, by examination of a section of a corpus luteum, on the basis 

 of distinguishing characteristics to be given below, in which one of the following periods of 

 pregnancy the specimen falls: 



Preparatory period 



Exoplasmic development, first part. . 

 Exoplasmic development, seeond part.. 



Transition period 



Endoplasmic development, first part 

 Endoplasmic development, second part 

 Beginning retrogression 



Length of 



Less than 20 

 20 to 30 

 30 to 55 

 55 to 140 

 140 to 170 

 170 to 220 

 220 to 290 



Needless to say, the corpus luteum, being an organic body, presents considerable 

 variety of structure, so that these periods overlap, and pass gradually into each other, in 

 such a way that, besides the specimens which fall near a border-line and hence are naturally 

 difficult to place, a small number give an appearance which experience shows is normal to 

 corpora lutea of older or younger age, just as in a series of embryos a few will exceed or fall 

 behind the others in development. 



CRITERIA OF AGE OF THE CORPUS LUTEUM. 

 First period: preparation.— Pigs less than 20 mm. long, duration of pregnancy less than 

 25 days (fig. 4). The corpus luteum of pregnancy does not attain full size until the 

 twentieth day or thereabouts, but variation in size is so marked that no dependence is to 

 be placed on dimension as a sign of early corpora lutea. On section the following features 

 may be noted : The tissue is very densely packed, so that the nuclei of the slender connective- 

 tissue cells seem relatively numerous. The septa of connective-tissue fibers are rather 

 marked, but are narrow and compressed, and still preserve the radial arrangement in which 

 they invaded the space. Between them are packed the lutein cells, which are polymorphic, 

 but usually elongated in the direction of the radius of the corpus luteum; they are much 

 more varied in size than in the succeeding stages. The nuclei are more chromatic than 

 they will be found later. The ring-forms occur in every lutein cell, and are so large and 



