The Corpus Luteum of the Guinea Pig 73 



D 3 of the corpora lutea in 25 unmated guinea pigs and in five 

 which were mated to a sterile male, and Fig. 2 is a similar 

 representation of the same organs in fifty pregnant guinea 

 pigs. The regression lines calculated from the data are given 

 in Fig. 3. 



The growth rate of the corpora lutea in mated and unmated 

 animals is very similar but the period of growth differs. In the 

 latter it ceases on about the 12th day and the organs then 

 rapidly regress ; the regression is not inhibited as the result of 

 sterile mating. In the pregnant animal the corpus luteum 

 continues to grow until the 18th to 20th day and is maintained 

 at this size for the rest of pregnancy, but after parturition it 

 rapidly becomes disorganized. Its involution occurs when the 

 new corpora lutea are actively growing in the same ovaries. 



Microscopic observations 



This investigation is not primarily concerned with detailed 

 cytological changes in the corpus luteum, but a general de- 

 scription of the more gross histological features of the ageing 

 organ is required since it is on these grounds that the distinc- 

 tion between the normal and the induced corpora lutea is 

 made. The mature follicle protrudes only slightly above the 

 surface of the ovary and after its rupture forms only a small 

 crater over which the germinal epithelium is re-established 

 in 48 to 72 hours. Both the granulosa and the theca interna 

 contribute to the luteal tissue which at three to four days 

 consists of a narrow band enclosing a central cavity as de- 

 scribed by Loeb. The cells are small and fairly closely packed ; 

 they have a small dark-staining nucleus surrounded by clear 

 cytoplasm of irregular shape. Mitotic figures are not common 

 at this stage. Possibly because of compression set up in the 

 cavity by the ingrowth of luteal tissue, haemorrhage occurs 

 very commonly and in some of the corpora lutea the cavity 

 becomes packed with red blood cells for as long as the 20th 

 day in some pregnant animals, but in others where bleeding is 

 less severe strands of connective tissue encroach and obliterate 

 the cavity by the 7th day or soon afterwards. The luteal cells 



