618 J. T. PATTERSON 



Particularly is this true of the changes which immediately follow 

 the attachment of the blastocyst. It is therefore proposed 

 briefly to describe some of those changes involved in the early 

 history of implantation and placentation which are essential 

 to a clear understanding of the embryology of the armadillo. 

 This account has been deferred until now, because it will be 

 .somewhat easier to follow after the description of the develop- 

 ment has been read. 



A statement as to the manner in which the embryonic vesi- 

 cle migrates along the horizontal groove of the uterus and even- 

 tually attaches itself has been given above. The principal 

 facts are as follows: After the blastocyst has reached the placen- 

 tal area, which lies at the extreme anterior tip of the fundus, 

 it adheres to the mucosa, apparently for some time before the 

 actual placentation or intimate union with the uterine mem- 

 brane is consumated. Four clear and two somewhat doubtful 

 cases of early adhesion between the ovum and the mucosa have 

 been observed. In every instance the ovum freed itself either 

 immediately or very shortly after the application of the fixing 

 fluid, thus indicating that the implantation proper had not 

 really taken place. Fortunately, these vesicles were studied in 

 salt solution under the binocular microscope before fixation and 

 it was possible to make out several important points. It was 

 observed that the ovum always comes to rest upon the uterine 

 surface in such a way that the embryonic spot or germinal area 

 is turned toward the mucosa. Hence, the foetal contribution 

 to the early placenta arises from that portion of the trophoblast 

 which overlies the inner cell-mass; that is, from the so-called 

 Rauber's layer. 



It is to this region of the trophoblast then that one must look 

 to detect the first indications of placentation. In five of the 

 blastocysts to which we have just called attention no evidence 

 of importance in this connection was observable, but in the sixth 

 unmistakable indications of early placentation are present. In 

 this specimen Rauber's layer is seen to have undergone impor- 

 tant modifications. Several of the cells are in mitosis, and all 

 of the cells of this region have lost their original attenuated 



