52 
GENERAL EMBRYOLOGY. 
the fold itself is the decidua capsularis or refiexa; the part on which the ovum 
rests is the decidua basalis or serotina, and the rest is known as the decidua vera. 
The decidua basalis lies in contact with the placental area of the chorion, 7.e. the 
Unchanged layer 
Stratum spongiosum 
Decidua basalis 
Amnion 
Stratum compactnm ss _> 
Cavity of. 
blastoderm 
Coelom * 
Villus 
Decidua vera 
i. a 
cidua capsular” 
y 
Yi 
Placental villus 
Maternal vessel 
Body stalk 
Allantoie 
diverticulum 
Eectoderm 
Somatic 
mesoderm 
Hntoderm 
Splanchniec 
mesoderm 
Decidua vera 
Fic. 39.—DrIAGrRaM, showing the completion of the decidua capsularis, the 
enlargement of the maternal blood-vessels in the stratum compactum of 
the decidua basalis, the increase of the placental villi, the formation of 
the amnion folds, and the appearance of the allantoic diverticulum. 
foetal part of the 
placenta, and it forms 
the maternal part of 
this organ. In the 
fully developed human 
placenta, the foetal and 
maternal tissues of 
which it is formed are 
so intimately mingled 
and blended together 
that it is impossible 
to say where one ends 
and the other begins. 
By a careful study, 
however, of a series of 
placentze of different 
ages a fairly clear and 
satisfactory idea of the 
part played by the 
maternal and _ feetal 
elements respectively, 
as well as of their 
relations to each other, 
may be obtained. The 
structural characters of the completed organ will be best understood if the two 
constituent parts are considered separately. 
vd 
The Fetal Part of the Placenta.—The villi of the placental portion of the 
chorion invade and 
penetrate the decidua 
basalis, whilst the vill 
of the non - placental 
chorionic area enter 
the decidua capsu- 
laris. 
As previously ex- 
plained, im connexion 
with the formation 
both of the amnion and 
of the chorion, the an- 
nular placental area 
is converted into a 
circular disc. It con- 
sists, ike the rest of 
the chorion, of ecto- 
derm and mesoderm, 
and it contains rami- 
fications of the allan- 
toic vessels; but the 
ectoderm is thickened 
and increased, its villi 
are larger than those 
of the non-placental 
Unchanged layer x 
Stratum spongiosum, 
Stratum compactum 
Placental villus (PZ 
Decidua basalis 
Amnion 
Maternal blood-vessel 
Maternal blood-sinus 
Cavity of § 
blastoderm 
Colom 
Villus 
Decidua vera 
Placental villus 
Allantoie 
stalk 
Body-stalk 
Allantoic 
diverticulum 
Ectoderm 
~_Somatic 
mesoderin 
Splanchnice 
mesoderm 
Entoderm 
Decidua vera 
Fic. 40.—D1aGram, showing enlargement of the blood sinuses in the maternal 
part of the placenta and the closure of the amnion. 
region of the chorion, and it is directly connected with the allantoic stalk. 
The early villi are merely ectodermal buds. 
They impinge against the surface 
epithelium of the decidua, which disappears before them. They then penetrate the 
sub-epithelial tissues, destroying and replacing the uterine elements. ach villus is 
