54 GENERAL EMBRYOLOGY. 
their terminations into large irregular spaces (Fig. 37). The increase of the 
interglandular tissue is most marked in the intervals between the dilated portions 
of the glands and their apertures, and when all the changes are fully established it 
is possible to re- 
cognise three layers 
Sa iieiiareees of decidual tissue 
‘ —(1) A superficial, 
“Maternal blood-sinus relatively thick 
layer, in which 
the interglandular 
tissue preponder- 
ates, the stratum 
compactum ; (2) A 
layer formed prin- 
cipally by the dil- 
ated portions of the 
Body-stalk glands, the stratum 
Allantoic spongiosum; and 
diverticulum (3) A thin deep 
portion of the 
pate] membrane which 
Sa eee contains the lower 
Decidua basalis 
Unchanged layer. ~~ 
Stratum : 
spongiosum —_ 
Feetal villus 
Stratum 
compactum 
Alimentary Uy 3 
canal Za, oe 17 Ectoderm 
Yolk-sac — Se extremities of the 
Splanchnic : ic 
Ble aes glands, which are 
Entoderm practically un- 
Ccelom 
changed, the un- 
changed layer. 
The decidua cap- 
sularis differs from 
the other portions 
of the decidua in 
Fig. 42.—D1acram, showing further growth of the placental sinuses and villi; the that it 1s only a 
fusion of the decidua capsularis with the decidua vera, and the obliteration of fold of the two 
the uterine cavity. Te 
superficial layers, 
and therefore it contains none of the deep “unchanged layer” of the decidua. 
Its central portion is formed by the stratum spongiosum folded upon itself, and 
its surfaces, external and internal, by the stratum compactum ; glands open upon 
each surface. 
The changes which occur in the decidua capsularis are due, first, to its connexion 
with and invasion by the chorionic villi; and, secondly, to the pressure exerted upon 
it by the enlarging ovum. The former influence is brought to bear whilst the 
decidua is still increasing; the latter, after it has reached its full development. The 
changes which result from its union with the chorion are the destruction and 
disappearance of the epithelium on its inner surface and the destruction and 
absorption of some of the interglandular tissue; they are due to the activity of the 
chorionic ectoderm which attacks and invades the uterine tissues. 
The changes due to the pressure exerted by the enlarging ovum are diminution 
of vascularity, disappearance of the lumina of the non-dilated portions of the 
glands, removal of the epithelium from the spaces of the spongy layer, compression 
of the spaces into slits, which ultimately disappear, and the coincident atrophy of 
the foetal villi, which have penetrated this portion of the decidua. All these 
changes result in the reduction of the decidua capsularis to a thin membrane in 
which no traces of the original structure are recognisable, in the fusion of the 
altered decidua capsularis with the decidua vera, and in the consequent obliteration 
of the uterine cavity. 
After the fifth month the decidua vera also undergoes atrophic changes, but 
they do not proceed so far as in the decidua capsularis; nevertheless the stratum 
compactum is greatly reduced, the superficial epithelium and the superficial parts 
of the glands entirely disappear from it, the interglandular tissue becomes less 
Decidua capsularis 
Decidua vera 
