368 MINOT. [Vou. II. 
ectoderm, 4, to be seen at eleven to thirteen days at the sides on 
the placenta, is a layer of mesoderm without any ccelom. Now, if 
my suppositions are correct, then the ectoderm forms at first an 
independent fold, da, beyond the terminal vein, vt; the meso- 
derm, but not the mesothelium, extends into this fold, which 
covers the sides of the placenta. The disappearance of the 
foetal ectoderm from the surface of the placenta, and the pene- 
tration of the foetal blood-vessels between the glands, are 
changes which take place during the eleventh day. How those 
changes occur, observations on the development at that age 
must decide. Meanwhile let us make shift with two hypothe- 
ses. The first is: The whole of the ectoderm attached to the 
placenta degenerates and is resorbed. Since the uterine epithe- 
lium, as observation indicates, has likewise disappeared from the 
placenta, the mesoderm, mes, of the allantois, a//, is brought into 
direct and free contact with the connective tissue and degener- 
ated glands of the placenta, and is thus enabled to carry by its 
own ingrowth the foetal blood-vessels into the very substance of 
the placenta. The second hypothesis is that the ectoderm and 
mesoderm have produced villi, which have grown into the pla- 
centa. In favor of this latter hypothesis there is certain evi- 
dence which I have not yet alluded to. In the deep portions of 
the glandular layer of the placentas of both eleven and thirteen 
one finds narrow loops of epithelium like a tuning-fork in shape ; 
the open ends of the U-loops are towards the top of the pla- 
centa; the epithelium composing them is a cylinder epithelium, 
which gradually thins out towards the upper end of the legs of 
the U;; it differs altogether in appearance from the degenerate 
gland epithelium, the interiors of the U’s contain vessels with 
foetal blood ; so far, then, these structures might be longitudinal 
sections of the ends of foetal villi. Towards the surface of the 
placenta the epithelium of the loops thins out, and I have not 
been able to follow them. If we have to do with villi, we must 
assume that the ectoderm has become exceedingly thin over 
their basal portions, but is preserved as a thicker layer over 
their tips, and my failure to trace the villi would be attributable 
to the imperfection of my preparations and observations. Bal- 
ancing the pros and cons leads me to favor the second hypoth- 
esis. Let me add that the mesoderm of the embryo is continu- 
ous without a break with the interglandular connective tissue ; 
