POLYEMBRYONIC DEVELOPMENT IN TATUSIA 623 



reason I have preferred to call it by the name of sub-Trager 

 cavity. 



A further examination of the figures on plate 7 will show that 

 at this stage of development the blastocyst is held to the muc osa 

 by the Trager zone alone, so that in order to free it from the 

 uterine wall it is merely necessary to cut this band of tissue at a 

 short distance beyond the edge of the vesicle. 



The transition from the Trager stage or primary placenta to 

 that of the secondary placenta, or a condition in which distinct 

 villi are present, is very gradual. The first villi arise from that 

 portion of the chorionic wall which corresponds to the Trager 

 zone; that is at the attached margin of the chorionic vesicle. 

 From here they successively appear further and further toward 

 the central area of the Trager epithelium, until its entire surface 

 becomes studded with them. However, the villi in the central 

 region do not become so long or so highly developed as those that 

 are situated towards its peripheral parts. 



Each villus starts as a thickening in the Trager epithelium, 

 and soon becomes a mass of cells protruding from the epithelial 

 surface. At first it is flat or disc-shaped and seems to serve as an 

 adhesive pad, but later it elongates into a true Trager cord, 

 which may become very much branched. These cords later 

 become invaded by a stroma-like mesenchyme, developed from 

 the mesodermal epithelium which directly overlies the Trager 

 epithelium. 8 



In order to understand the changes which take place in the 

 upper free portion of the chorionic wall it is necessary to recall 

 that all of the trophoblast lying above the margin of the ento- 

 derm sooner or later sloughs off, leaving only a fragment-like 

 base, which in section can often be seen protruding from tne 

 side of the vesicle at a short distance above the uterine surface. 

 This fragment is clearly shown in all of the figures of plate 9. 



The portion of the trophoblast which thus breaks away cor- 

 responds to the chorionic ectoderm in the vesicle of other mam- 



8 For a more detailed account, and for figures of the structure of the villi, see 

 Newman and Patterson, '10. 



