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 is that 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 



