404 MINOT. [Vou. Il. 
derm of the placental chorionic mesoderm undergoes patchwise 
manifold changes ; it exists in three chief forms: 1°, the nucleated 
protoplasm ; 2°, the cellular layer; 3°, canalized fibrine. A patch 
of the ectoderm may consist of any one of these modifications, 
of any two or of all three, but they have fixed relative positions, 
for when the nucleated protoplasm is present, it always covers 
the free surface of the chorion; when the cellular layer is 
present, it always lies next the mesoderm; and when all three 
forms are present over the same part, the fibrine is always the 
middle stratum. In general terms it may be said that the 
amount of canalized fibrine increases with the age of the 
placenta, but it is very variable in its degree of development. 
The peculiar layer into which the ectoderm is transformed has 
long puzzled anatomists. E. H. Weber recognized the fibrine 
layer and described its appearance correctly; it has probably 
been often seen, but generally regarded as either pathological 
or a blood coagulum. Robin, for instance, may be cited, 125, 
70-71, as one who saw, without observing correctly and under- 
standingly, the tissue in question. An important gain was 
made when Winkler recognized the modified ectoderm as a 
constant layer, and in 1872 directed especial attention to it 
under the name of “ Schlussplatte,’ 152. Kolliker (Entwicke- 
lungsgeschichte, 2te Aufl., 337) added essentially to our knowl- 
edge of its structure, but it is to Langhans that we owe the first 
clear light. Meanwhile, other writers, following the lead of 
Ercolani and Turner, 146, 551-553, have been influenced chiefly 
by the presence of the cellular layer, in the large size of the ele- 
ments of which they found a resemblance to the decidual cells, 
which has guided them to the conclusion that the cellular layer 
is derived from the wall of the uterus. This error has been 
definitely corrected by Kastschenko, as already stated. In 
further support of the conclusion that the chorionic cellular 
layer is not decidual, may be brought forward the fact that 
there is a certain immigration of decidual cells into the placenta 
at its margin; but they remain entirely distinct from the cells 
of the cellular layer. This is readily seen in radial sections 
through the margin of a placenta from a normal after-birth — 
compare below, the account of the ectoderm of the chorion 
leve. The origin of the canalized fibrine from blood, which 
Langhans left in his first paper as an open possibility, and which 
