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 191 1, 



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 tune 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 



