THE OESTROUS CYCLE IN THE MOUSE 311 
the central cavity in only one of the eight corpora lutea of this 
set. From the position of the ova in the tubes, it was concluded 
that ovulation has occurred three days previous to killing. This 
set of corpora lutea of oestrus is, therefore, three days old. They 
are surely the equivalent of the seventy-two-hour corpora lutea 
of pregnancy. 
The next older corpora lutea are greatly degenerated. They 
are small, deeply placed, irregular in outline, and poorly demar- 
cated from the surrounding stroma. The proportion of connec- 
tive-tissue cells to luteal cells is about equal. As stated above, 
the cytoplasm of the luteal cells stains red with eosin, but it is a 
faded or blotchy red. No distinct large blood vessels are present 
as in the first set described. A third set consisting of only a 
few still more poorly defined corpora lutea is present. These 
are very small, staining a light pink, and are almost completely 
obliterated by a connective-tissue ingrowth. 
Of the three sets of corpora lutea distinguishable, the age of 
the youngest can be placed at three days, with a limit of error 
of about twenty hours. From the results of the examination 
of other ovaries of this series, the second set is at least ten days 
old, and the third still older. 
There are a few cells present in these ovaries which could be 
defined as interstitial. 
The prooestrum 
Animal *9 was chosen as typical of the stage P condition. 
A routine examination was made at 1 p.m. and another just 
before killing at 4.30 P.m., an interval of three and one-half 
hours, in which time many of the leucocytes had disappeared 
from the vaginal contents, indicating an early stage P. Pre- 
vious to killing she had been observed for sixteen days, during 
which time four complete cycles had been recorded, making the 
average duration four days. External signs were marked dur- 
ing all four of the oestrous periods. These occurred 4, 7 to 8, 
11 to 12, 15 to 16 days before death. Her record is one of mini- 
mum cycle duration and perfect regularity. 
