326 EDGAR ALLEN 
Corpora lutea of oestrus in the mouse differ in no details 
distinguishable histologically from those of pregnaney during 
the first four days of development at least. In only a few cases 
is ‘‘bleeding into the central cavity” found. Three days are 
required for the hypertrophy of the former granulosa cells and 
the ingrowth of the theca interna to completely fill the central 
lake of tertiary liquor folliculi. For five or six days (unless 
another ovulation intervenes) these newly formed corpora stain 
blue with haematoxylin, and are therefore distinguishable until 
the next oestrus. After this time they have an affinity for eosin. 
If size is taken as a criterion of development, corpora lutea of 
oestrus in the mouse do not attain their maximum until an age 
of from ten to fourteen days is reached. This usually corre- 
sponds with the second ovulation after the one initiating their 
growth as corpora lutea. Even at the third ovulation following 
their start they may be equal in size to five-day corpora (fig. 24). 
Thus, an ovary containing from ten to sixteen large, clearly de- 
fined corpora lutea, the result of three ovulations between the 
fifth and sixteenth days preceding, may again ovulate. After 
twenty days, at which time they are usually not superficially 
located, ingrowth of cells from the stroma obliterates their 
outlines. 
In animals of the first class, then, there are always present in 
the ovaries one recent set of blue-staining and two, three, or 
four older sets of red-staining corpora lutea, and normal follicles 
of medium to large sizes. In the O period follicles attain their 
largest size, and the youngest set of corpora are solid and blue 
staining. In the M, stage the former corpora stain red and a 
more recent set of blue-staining ones are developing in the rup- 
tured follicles. At this time (one day after ovulation) they 
contain large central lakes of tertiary liquor folliculi (fig. 22). 
The largest follicles are medium sized and liquor is forming at 
their poles. During the M2 stage the ovary is not subject to 
the general leucocytosis occurring in the uterus and vagina at 
this time. In the middle of the D interval the blue corpora are 
solid and the largest follicles contain one fairly large lake of 
primary liquor folliculi. During the P stage the follicles become 
