The Corpus Luteum of the Guinea Pig 77 



field upon which classification was based. A very close agree- 

 ment between classification based on the results of micro- 

 scopic examination and size of the gland was obtained. The 

 most reliable criterion used to separate the two sets of glands 

 was the degree of vascularity but when at a later stage of 

 development closure of the blood vessels had occurred in the 



Stage in pregnancy (days) 



Fig. 4. Mean D 3 in mm. 3 of corpora lutea induced on the 8th (O) ; 21st ( X ) and 



35th day (0) of pregnancy compared with the normal structures in unmated 



and pregnant guinea pigs. 



induced corpora lutea as well, the distinction was made on 

 cell size, nuclear shape, vacuolation of the cytoplasm and the 

 estimates of the numbers of mitotic figures present. The data 

 obtained are presented in Table 2 and Fig. 4 on which lines 

 fitted by eye, illustrate the trend of growth of the corpora 

 lutea induced at the three different stages of pregnancy. The 

 first wave of follicles to mature, when ruptured artificially 

 on the 8th day of pregnancy, form very small corpora lutea 

 growing at a very slow rate, but although they do not attain 

 the size of those in the unmated animal, they are maintained 



