No. 636] POLYEMBRYONY IN THE ARMADILLO 65 



obtain the cleavage stages, although judging from the condition of devel- 

 opment in the vesicles collected in previous years, one would naturally 

 expect to find these early stages during the period of my first collec- 

 tion in 1911. 



The following year he began collecting still two weeks earlier 

 and again had a similar experience. 



Practically all of these vesicles lie free withm llie uterino cnvitv, 

 either in the horizontal groove or in the region of the aftaehiiipnt 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, pos- 

 sesses a free blastocyst. . . . Taking all the facts into consideration, I 

 estimate the "period of quiescence" to last about three weeks; that is, 

 from about the middle of October to the third or fourth of November. 



In a study of sections no mitotic divisions were found to occur 

 in the blastocysts during the ' ' quiescent period. ' ' 



The only point of interest cited by Patterson in connection 

 with this peculiar phenomenon of interruption in development, 

 was the fact that in no other mammal, except the deer, had 

 such a condition been found. Bischoff had long ago, 1854, re- 

 ported a "period of quiescence" lasting for some weeks during a 

 so-called morula stage of the deer embryo. 



Newman* has recognized the importance of Patterson's dis- 

 covery of a ''quiescent period" during the early development of 

 the armadillo, and states in a discussion of twin formation that 

 this "period of quiescence" probably, "holds the clue to the 

 physiological explanation of polyembryony. " In this position 

 Newman is, in my opinion, largely right, but this is as far as the 

 data led him, and he finally remarks : 



The problem is to locate the factors responsible for the slowinir down 

 of the develnpnienfal rhythm. Whatever these factors may be, and 



Newman thus fails to appreciate the second point in Patter- 

 son s discovery, and that is that the blastocysts always lie free 

 in the uterus during the "period of quiescence." This fact en- 

 ables us to go one step further since the lack of attachment and, 

 therefore, lack of oxygen supply are very probably "the factors 

 responsible for the slowing down of the developmental rhythm." 



