66 LEO LOEB 
difference consists in a prolongation of the sexual cycle, which lasts 
as long as the pregnancy continues. The changes in the follicles are 
identical with those found in the ordinary sexual period not 
accompanied by pregnancy. 
After copulation the period of growth following the sudden 
degeneration of the follicles is the same as in the ordinary sexual 
period, but the period of follicular equilibrium is much pro- 
longed. 
During this period of follicular equilibrium certain follicles 
ean not only grow to a considerable size, but may even undergo 
the additional changes which indicate the maturation of the folli- 
cle. A rupture of follicles does not however take place during 
pregnancy under ordinary circumstances. 
The corpus luteum of pregnancy differs from the ordinary 
corpus luteum mainly in its prolonged duration of growth and of 
life. At a time when, in the ordinary corpus luteum not accom- 
panied by pregnancy, mitoses have ceased to be present and the 
retrogressive changes are very marked, mitoses are still seen in the 
corpus luteum of pregnancy. In the corpus luteum of pregnancy 
degenerative changes set in before the end of pregnancy has been 
reached, and they continue after delivery. A short time after 
delivery a new ovulation usually occurs, even if no copulation had 
taken place after delivery. The retrogression of the corpora lutea 
of pregnancy continues, but it requires much more time than the 
retrogression of an ordinary corpus luteum. 
The mechanism that governs the sexual cycle in the ovary can 
be recognized only incompletely by observation and it has been 
the subject of an experimental investigation, the results of which 
we shall report in more detail elsewhere. We may however state 
that our experiments have shown that through extirpation of the 
corpora lutea the sexual cycle is shortened. The presence of well 
functioning corpora lutea inhibits a new ovulation. Pregnancy as 
such does not prevent ovulation. Ovulation can be made to 
take place even in pregnancy, if the corpora lutea be extirpated 
at an early period after copulation. And under such conditions 
the typical follicular changes follow the ovulation during preg- 
nancy. As soon therefore as degenerative processes have set 
