TWINNING IN DASYPUS NOVEMCINCTUS 39 



obtain the late cleavage and early embryonic stages, 

 and to his paper' of 19 13 we are indebted for a 

 large part of the following account of this period of 

 development. 



The earliest stages found were collected on October 

 15; these consisted of two eggs in the Fallopian tubes, 

 and several eggs floating freely in the uterine cavity. 

 In no case was more than one egg found in the uterus 

 or tubes of one female. From the fact that the Fallopian- 

 tube eggs and all those found free in the uterus were in 

 almost precisely the same embryonic stage, and from 

 the additional fact that nearly every large female 

 examined as late as three weeks after the earliest date 

 mentioned had an egg in practically the same stage of 

 development, it must be concluded that there is a 

 period of quiescence of about three weeks, during which 

 the egg either remains at a standstill or else develops 

 so slowly as to make no perceptible progress. Although 

 Patterson draws no conclusion as to the effects on 

 development of this period of quiescence, it is my behef 

 that this period holds the clue to the physiological 

 explanation of polyembryony. Consideration of this 

 point is deferred, however, until the general discussion 

 of the underlying causes of twinning. 



The following account of the embryology of Dasypus 

 novemcinctus I have built up for the convenience of the 

 reader around a number of carefully constructed ligures; 

 although slightly diagrammatic, they are accurate in 

 all essential points, and are certainly more intelligible 

 than microphotographs, or exact copies of preserved 

 and sectioned material. I have found that zoologists 



^ J. T. Patterson, Journal of Morphology, XXIV (1913). 



