STRUCTURE AND DEVELOPMENTAL RATE 193 



about three weeks; that is, from about the middle of October to 

 the third or fourth of November. ... Of the thirty-four 

 free blastocysts obtained in 1911 and 1912, twenty-eight of them 

 were secured within this period." 



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 ('54) had long ago reported a 

 'period of quiescence' lasting for some weeks during a so-called 

 morula stage of the deer embryo. 



Newman ('17) has recognized the importance of Patterson's 

 discovery of the 'period of quiescence' during the early develop- 

 ment 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 slowing down of the develop- 

 mental rhythm. Whatever these factors may be, and we have no 

 definite knowledge of them, the result of retardation is polyem- 

 bryony." 



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." 

 The armadillo egg, like that of most mammals, undergoes its 

 early development in the Fallopian tube and is, therefore, capable 

 of reaching the blastocyst stage on its initial oxygen supply. After 

 this time however, it must become attached to the uterine wall 

 for a further source of oxygen. For some reason, in the armadillo 

 the reaction between the blastocyst and the uterine wall is post- 

 poned and the blastocyst is incapable of further developmental 

 progress until this reaction is established and the necessary supply 



