371 



blast at the animal pole continues to remain distinct from the under- 

 lying layers (Fig. 2). 



Upon reaching the placental zone, the vesicle, which has now 

 attained a diameter of about 430 mm, becomes attached to the 

 uterine mucosa^ of this area. At first the attachment could be more 

 properly spoken of as simply an adhesion. There is here a slight 

 break in my series, and I am therefore not able to speak with con- 

 fidence concerning the changes that immediately follow the attachment 

 of the vesicle; but from what takes place later, it seems clear that, 

 in the strict sense 

 of the word, an 

 „implantation" of 

 the egg does not 

 take place. The 

 attachment of the 

 egg is apparently 

 followed immedia- 

 tely by the forma- 

 tion of the Träger, 

 or primitive pla- 

 centa, which arises 

 from that portion 

 of the tropho blast 

 which comes di- 

 rectly in contact 

 with the mucosa. 

 The Träger is soon 

 differentiated into 

 two rather distinct 

 regions One of 

 these occupies the 

 central part of the 

 attachment area 

 and is characte- 

 rized by retaining 

 for some time the 



primitive condition of the trophoplast ; the other is a zone imme- 

 diately surrounding the former and representing the outer border of 

 the attachment area. This zone is conspicuous for the thickened 



24* 



*#* 



' /■'^^tV:-'^^*^,'li' 



■>4t^,. 



Fig. 2. Section through the embryonic spot of a vesicle 

 measuring .429 mm. in diameter. This vesicle was just be- 

 ginning to adhere to the mucosa. 



Fig. 3. Photograph of a median section of a vesicle in 

 which the process of " germ layer inversion " has just been 

 com-pleted. E, cavity of the ectodermic vesicle ; Ta, Träger 

 portion of the trophobiast ; EN, entoderm. 



