POLYEMBRYONIC DEVELOPMENT IN TATUSIA 569 



very late in the breeding season, one (No. 268) on November 14, 

 and the other (No. 284) on December 12. 



In table 2 are listed all the early attached or implanted vesicles 

 which are directly referred to in the subsequent parts of the paper. 

 They are arranged, so far as possible, in the order of their degree 

 of development. Vesicle No. 311 of the preceding table should 

 probably be included in this list, and No. 300 should almost cer- 

 tainly be included. They were both lying free upon the placental 

 area when first observed, and for that reason are included in the 

 list of unattached specimens; but inasmuch as the early attached 

 vesicles are so easily dislodged, it might well be that these two 

 vesicles were loosened during the process of everting the uterus. 

 Their size and structure most certainly point to this conclusion. 



In determining the size of those vesicles which had gained a 

 firm attachment to the mucosa it was found best to measure the 

 base of the vesicle, as this gave the most trustworthy results. In 

 everting the uterus, however, the mucosa of the placental area 

 may become slightly stretched or otherwise distorted, thus spread- 

 ing the base of the vesicle, and consequently giving a measurement 

 that is abnormally large. This is the case in No. 256. Further- 

 more, all of the vesicles listed beyond No. 332 were measured from 

 the microscopic preparations, and consequently after the shrink- 

 ing effects of dehydration, clearing, and imbedding had come in. 

 The sizes indicated for these vesicles are too small as compared 

 with those listed above No. 332 (cf. 233 and 307). 



DEVELOPMENT OF THE BLASTOCYST 

 1. The monodermic blastocyst 



The transformation from the so-called morula stage to the hol- 

 low sphere, or 'monodermic blastocyst/ has not been observed in 

 the armadillo. The youngest normal stage which I have at my 

 disposal already shows the finished product of this transforma- 

 tion. However, I possess representative stages throughout the 

 period of development which extends from this point on until the 

 'didermic blastocyst' is reached. In the present section we shall 

 describe three blastocysts, which, although all of the monodermic 



