564 J. T. PATTERSON 
The breeding season extends over a considerable portion of the 
months of October and November, and thus any lot of embryos 
taken at a given time during the period of gestation will present 
much variation in development. This makes the determination 
of the length of gravidity difficult and quite uncertain. An ex- 
act determination could only be made by breeding animals in 
captivity. Since a majority of the young are born in the months 
of March and April, gestation is probably about one-hundred and 
forty days. 
The old females breed first, mating in most cases before October 
fifteenth, while the second year virgin females continue to breed 
for some time after this period. Females of one year do not breed 
except in rare instances. My records for the past five years show 
just three cases of pregnancy among these young animals, out of at 
at least two-hundred examinations. 
In connection with the collecting of material and this incidental 
study of habits, I have discovered a ‘period of quiescence’ of 
the embryonic blastocyst. The fact was first made apparent in 
1911, when, after I had started collecting two weeks earlier than 
in the preceding year, I failed to obtain the cleavage stages, al- 
though judging from the condition of development in the vesicles 
collected in previous years, one would naturally expect to find 
these early stages during the period of my first collections in 1911. 
Again in 1912, I began collecting material two weeks earlier 
than in 1911, and much to my surprise obtained blastocysts in al- 
most exactly the same condition as those secured during the pre- 
ceding fall. Practically all of these vesicles lie free within the | 
uterine cavity, either in the horizontal groove or in the region of 
the attachment zone (placental area). 
It is evident from these data that the embryonic vesicle remains 
for some time lying free within the uterine cavity. Just how long 
this period lasts, I am unable to state; for practically every old 
female taken at the earliest date (October 15) at which I have 
collected, possesses a free blastocyst. How long such blastocysts 
have been in the uterine cavity it is, of course, impossible to 
determine; but I should judge not very long, because two vesicles 
