POLYEMBRYONIC DEVELOPMENT IN TATUSIA 621 
In the next stage, which is but slightly older than No. 316, a 
somewhat larger number of nuclei have passed over into the 
uterine wall. They have almost completely destroyed the epi- 
thelium, and have also affected the sub-epithelial layers (fig. 17). 
The syncytium and adjacent thickened trophoblast together form 
a layer which is coextensive with the germinal spot, that is, they 
extend to the margin of the entoderm. In addition there is 
found on the left wall, and somewhat removed from the rest of 
the thickened extragerminal ectoderm, a distinct knot of cells 
which must have resulted from the contact of a very small spot 
of the trophoblast with the mucosa (fig. 17, K). 
The next important change is a spreading of the base of the 
vesicle. Instead of having but a small area of contact or point 
of union, as in specimen Nos. 316 and 332 (figs. 31 and 17), the 
blastocyst soon acquires a base, often greater in width than that 
of the entire free portion of the vesicle (fig. 18). As a result, 
the wall of the chorion, upon approaching the mucosa, instead 
of curving inward, now slopes gradually outward, and merges 
into the uterine tissue with which it has established a very — 
intimate union. The base forms an annular zone of thickened 
trophoblast, which in width extends from the margin of the en- 
toderm to the surface of the uterine wall. In its more distal 
part the annular zone consists of thickened cells (two or three 
cells thick), but its proximal part is much thicker and one finds 
here a tendency to form a syncytium, continuous with the 
syncytial-like mass lying directly beneath the chorionic vesicle. 
In the latter region are found numerous embryonic nuclei, 
which have destroyed the mucous epithelium, the connective- 
tissue stroma, and even portions of the uterine glands. These 
nuclei may be divided into two groups, one of which occupies a 
superficial position; that is, its elements bordér on the extra- 
embryonic cavity and apparently are inactive in so far as the 
- destruction of the mucosa is concerned; and the other consti- 
tutes the deeper lying nuclei which are instrumental in dissolv- 
ing the mucosa. At first this distinction is not very evident, 
(figs. 18 and 32), but as development progresses the protoplasm 
surrounding the surface nuclei becomes more and more dense 
