POLYEMBRYONIC DEVELOPMENT IN TATUSIA 585 
Measurements show that the entoderm has practically reached 
its maximum extension in such specimens as No. 311, in which the 
embryonic spot measures 0.020 mm. by 0.157 mm.., and covers an 
arc on the circumference of 36°, 18’. It does not here reach much 
beyond the extreme limits of the ectoderm (fig. 11). This does 
not mean that the entoderm may not cover an actually great 
area, for it becomes attenuated through the expansion of the 
trophoblastic wall; but what is meant isthat the entodermal cells 
do not push out any further along the trophoblastic wall. Con- 
sequently that part of the wall which lies beyond the limits of 
the embryonic area never becomes didermic, or bilaminar. Per- 
haps then it would be better to confine the use of the term dider- 
mic to the embryonic spot, and not apply it to the blastocyst as 
a whole. . 
It may not be amiss in conclusion to draw attention again to the 
very close similarity between the mode of origin of the entoderm 
in the armadillo and that of Dasyurus, as given by Hill. The 
similarity is indeed striking, especially if one consider the funda- 
mental differences in the character of the walls of the two types 
of blastocysts. The thin wall of the formative region of the blast- 
ocyst of Dasyurus stands in sharp contrast to its homologue, the 
relatively thick inner cell-mass of the armadillo blastocyst, and 
yet in all essential features their modes of entoderm formation are 
practically identical. 
ATTACHMENT OF THE BLASTOCYST 
The implantation of the embryonic vesicle is a subject of much 
importance, and is treated elsewhere as fully as the material at 
hand warrants. At this point it is desirable to discuss briefly only 
the first step in implantation, or the attachment of the blastocyst. 
A great deal of effort has been put forth to obtain the earliest 
attached stages, and, to date, four clear cases have been observed. 
Two other doubtful cases were seen. Unfortunately in each case 
the vesicle became detached from the mucosa upon placing the 
uterus in the fixing fluid or very shortly thereafter. In two of 
these the separation took place immediately, while in the other two 
from two to three minutes elapsed between the immersion of the 
