SEPTEMBER 13, 1918] 
ence with the uterus which was practised in 
these cases might somewhat modify the dura- 
tion of the sexual cycle, but that if such a 
modification did exist, it applied equally to ex- 
periments in which the corpora lutea had been 
extirpated and to control cases. Therefore our 
conclusions concerning the inhibiting func- 
tion of the corpus luteum was not affected by 
such an interference. However, we had in- 
tended to continue our investigation in this di- 
rection and in later determinations we found, 
in guinea-pigs in which the uterus had not 
previously been interferred with, the length 
of the sexual period to vary between 15 and 
18 or 19 days. In two cases we observed the 
new ovulation as early as 133 to 143 days after 
the preceding ovulation. A certain latitude 
exists therefore in the periodicity of the nor- 
mal sexual cycle. 
Again we could confirm our previous results: 
Excision of the corpora lutea carried out 
within the first week after copulation brings 
about a new ovulation between the tenth and 
fifteenth day after copulation in the majority 
of such cases, and we were able to observe it as 
early as 84 and 9 days after copulation. 
As in our previous experiments pregnancy 
did not prevent the early ovulation after a 
complete extirpation of the corpora lutea. It 
is only the persistence of the corpora lutea of 
pregnancy which prevents ovulation. 
While in our previous investigations we had 
studied mainly the cyclic changes in the 
ovaries and only incidentally referred to cyclic 
changes, in the uterus, in the last-named 
paper we extended our studies to the cyclic 
changes in the uterus and to the correlation of 
the cyclic changes in the uterus and ovaries. 
In our determination of the sexual cycle we 
made use of the following criteria: 
1. Whenever feasible the period of heat was 
observed; the behavior of the animal as well as 
the condition of vulva and vagina serving as 
indicators, which in the hands of an experi- 
enced breeder gave reliable results as the sub- 
sequent macroscopic and microscopic examina- 
tion of uterus and ovaries proved in each case. 
8 Leo Loeb, Biological Bulletin, 1914, XXVII., 
p. 1. 
SCIENCE 
275 
In other animals the observed copulation indi- 
cated the approximate time of ovulation. We 
followed the cyclic changes in uterus and 
ovaries from day to day and could thus estab- 
lish a definite and orderly sequence of events. 
By correlating the condition of the corpora 
lutea, follicles and uterine mucosa, it was pos- 
sible to determine the stage of the sexual cycle 
of the observed guinea-pigs with a precision 
which was entirely adequate for our purposes. 
Thus to mention only one instance it was pos- 
sible by microscopic examination of the uterus 
alone without examination of the ovaries to 
determine whether in an animal at an early 
stage of the sexual cycle ovulation had or had 
not ‘taken place. 
Subsequent to our last mentioned investiga- 
tions, Stockard and Papanicolaou® published 
a study of the sexual cycle in the uterus of the 
guinea-pig, in which they used the naked eye 
observations of vaginal changes as a criterion 
of the stage of the sexual cycle. These investi- 
gators confirmed in all essential points our 
previous results, though they do not mention 
our principal paper in which we gave a de- 
tailed description of the uterine cyclic changes 
in the guinea-pig. This omission, Dr. Stock- 
ard informed me later, was due to his failure to 
read my paper. 
The difference in the duration of the sexual 
cycle in our first and second series of experi- 
ments led to a further elucidation of the fac- 
tors on which the mechanism of the sexual 
cycle depends. We found that the presence of 
experimentally produced deciduomata with- 
out accompanying pregnancy prolongs the 
duration of the sexual cycle, but only in cases 
in which the corpora lutea are present. After 
extirpation of the corpora lutea an accelerated 
ovulation takes place notwithstanding the 
presence of living deciduomata; the deciduo- 
mata act, therefore, in a way similar to preg- 
nancy. We found furthermore that after de- 
generation of the deciduomata ovulation oc- 
curs. We may therefore conclude that living 
deciduomata probably prolong the life of the 
corpora lutea and that this effect leads to a 
® Charles R. Stockard and G. N. Papanicolaou, 
Am, Jour. Anatomy, 1917, XXII., p. 225. 
