No. 3.-] UTERUS AND EMBRYO. 377 
only by the ectoderm, to which the mesoderm soon joins itself. 
As soon as the coelomatic fissure appears, we can speak of a 
foetal chorion adhering to the placenta. When the allantois 
grows out, it forms the stalk of connection between the embryo 
and the placental chorion. After the development of the cho- 
rion, the free surface of the placenta is, of course, covered by 
mesothelium (the epithelium of the coelom). Outgrowths of 
the chorion penetrate the glandular layer of the placenta; 
whether these outgrowths are in the form of villi in the sense 
that they preserve a covering of foetal ectoderm was not ascer- 
tained, although the tips of the outgrowths appear to have such 
a covering, and there is no mingling of the foetal with the ma- 
ternal circulation. The ccelom of the embryo does not extend 
to the edge of the placenta next the peri-placenta, but the meso- 
derm does, and is covered by ectoderm. 
In the peri-placenta, the glands degenerate and disappear com- 
pletely, but the covering epithelium is reconstituted except on 
the part near the placenta. The blood-vessels and connective 
tissue change as in the outer zone of the placenta, though later. 
At the fifteenth day a few young monster cells were found near 
the surface. 
In the ob-placenta the degeneration and resorption affect only 
the surface epithelium and the upper part of the glands; the 
deep portions remain as a series of shallow cups, having been 
stretched transversely by the expansion of the ob-placenta; the 
epithelium of the cups unites into a new continuous layer ; 
the glands grow up into follicles and are again stretched out by 
the expansion of the walls. Meanwhile there appear monster 
cells, which probably arise by the hypertrophy and migration of 
single cells of the epithelium; they are characterized by the 
granular hyaline appearance of their bodies, by the coarse gran- 
ulation and large scattered fragments of chromatine of their 
nuclei, and by their hugeness. The monster cells continue to 
enlarge and subsequently invade the whole thickness of the 
annular muscularis. They appear first and are always most 
numerous directly opposite the placenta, but they are ultimately 
present throughout the ob-placenta. 
The relations of the embryo having been outlined in § 8, with 
the aid of Plate XXIX., it is not necessary to recapitulate them 
again. 
