POLYEMBRYONIC DEVELOPMENT IN TATUSIA 565 



taken from the fallopian tubes show a development almost as far 

 advanced as that of some vesicles taken from the proximal -parts 

 of the horizontal grooves. 



Taking all the facts into consideration, I estimate the 'period of 

 quiescence' to last about three weeks; that is from about the mid- 

 dle of October to the third or fourth of November. There are ex- 

 ceptions to this, but not sufficient in numbers to modify the gen- 

 eral conclusions. Of the thirty-four free blastocysts obtained in 

 1911 and 1912, twenty-eight of them were secured within this 

 period. 



There is another line of evidence which more or less supports the 

 above conclusion. I refer to the condition of the corpus luteum. 

 In all the females from which the free blastocysts were taken the 

 corpus has attained approximately its maximum size. Unless we 

 attribute a phenomenal rate of growth to the corpus, it is neces- 

 sary to assume that quite a long period has elapsed since ovulation 

 took place, in order to account for its large size. The short fal- 

 lopian tube of the armadillo precludes the suggestion that the egg 

 has spent a great while in traversing this passage, so that we must 

 conclude that any arrested development in the egg takes place 

 after it enters the uterine cavity. 



So far as the writer is aware, the only other mammal in which a 

 similar quiescent period in the development of the blastocyst oc- 

 curs is the deer. According to Assheton ('98) Bischoff ('54) states 

 that in this animal the embryo, upon reaching the so-called morula 

 stage, enters upon a 'period of quiescence,' and remains unaltered 

 for some weeks. 



It is scarcely correct, however, to state that the blastocyst of 

 the armadillos, during the entire period of quiescence, is in a state 

 of arrested development, because it undergoes certain progressive 

 changes. So far as one can tell from a study of sections, no mi- 

 totic divisions occur, but the vesicle increases in size, due to the 

 accumulation of fluid within its cavity, accompanied by the atten- 

 uation of the trophoblastic cells. Furthermore, the differentia- 

 tion of the ectoderm and entoderm from the inner cell-mass is com- 

 pleted while the blastocyst lies free within the uterine cavity. 



