POLYEMBRYONIC DEVELOPMENT IN TATUSIA 619 
appearance, and now show a decided increase in thickness (fig. 
11). There is also a distinct tendency for some of the cell 
walls to disappear, thus transforming Rauber’s layer into a 
syncytium. Furthermore, the surface of Rauber’s layer presents 
a ‘fuzzy’ appearance, the outer wall of some of the cells actually 
being broken as though ruptures had resulted when the blasto- 
cyst was freed from its insecure moorings to the mucosa. 
That all of these facts are evidence of the beginning of im- 
plantation is clearly indicated by the act that the trophoblastic 
cells which lie beyond Rauber’s area are still unchanged, and 
show the mosaic-like arrangement of polygonal cells so charac- 
teristic of all of the free blastocysts. Unfortunately, there is 
here a slight break in the series, so that we are not able to fol- 
low up this clew through the obviously critical period of implan- 
tation. We are therefore obliged to pass directly to an ac- 
count of the modifications which are occurring both in the 
mucosa epithelium and in the wall of the blastocyst in a vesicle 
which has already become firmly anchored to the maternal 
tissue. 
Blastocyst No. 316 represents the youngest firmly attached 
stage that has been secured. A series of sections from this 
specimen is shown in plate 2. In the living vesicle it could be 
seen that, in addition to the small area which had established 
an intimate union with the mucosa, almost the entire left side 
was lying in contact with the uterine wall, and that as a result 
the trophoblastic cells here had greatly increased in thickness 
(fig. 16). Whatever may be the nature of the stimulus which 
causes the trophoblastic cells to react whenever brought into 
close relation with the uterine epithelium, it is certain that the 
influence is not confined to Rauber’s layer, but any portion of 
the trophoblast, upon coming in contact with the mucosa, will 
respond. It does not necessarily follow that such thickened 
trophoblast establishes eventually a fusion or placental union 
with the uterine wall, although a study of the stages more ad- 
vanced than this one, suggests that some of it may so unite. 
However, it should be pointed out that only in rare instances 
does a blastocyst become attached in a manner such that an 
JOURNAL OF MORPHOLOGY, VOL. 24, No. 4 
