THE PLACENTA. 53 
at an early period penetrated by an outgrowth of the subjacent mesoderm carrying 
branches of the allantoic vessels, and so it becomes vascular. For some time all 
the villi, placental and non-placental, grow and absorb nutriment from the maternal 
tissues, probably utilising as food the tissues which they destroy and replace; but 
when the decidua capsularis is thinned by the expansion of the growing ovum, the 
villi of the non-placental region which have penetrated it are no longer able to 
obtain nutrient matter, and they consequently atrophy and disappear. The 
placental villi, on the contrary, continue to increase; they grow in size and become 
more complex, and, anastomosing together, enclose the dilated maternal blood-vessels. 
The walls of the 
latter disappear, VBE 
and secondary State ~ 
branches growing SORES 
oy 
from the foetal villi z go Allantoie stalk 
project into the y 7, Wy, 22 _ oay-stalk 
Decidua basalis 
Maternal blood-sinus 
VA A Maternal vessel 
7 Placental villus 
Placental villus 
——_ 
enclosed spaces CS ~ 
and floatinmater- =ss_77 > i: 
nal blood. When ~ Se /j 
the formation of 
the placenta is 
completed, its foetal amnion 
part consists of 
villi, each of which 
possesses an ex- Celom 
ternal covering of 
two layers of ecto- 
dermal cells and a 
vascular meso- 
dermal core; the 
villus projects into 
the interior of a 
large blood space, 
which is_ sur- 
rounded, more or Fic. 41.-—Diacram, showing the fetal ectoderm surrounding the maternal blood 
less completely, by sinuses, the commencement of secondary foetal villi which project into the 
Patel Pender sinuses, and the disappearance of the superficial portions of the glands. 
> 
and it is bathed by maternal blood from which it obtains the materials necessary 
for the nutrition and growth of the embryo, and into which it transmits effete 
excretory matter from the embryo. 
The Maternal Part of the Placenta and the Changes in the Decidua.— 
The occurrence of further changes in the decidua, after the developing ovum enters 
the uterus, is dependent upon the retention of the ovum in the uterine cavity. 
These changes, therefore, only occur in what may be appropriately termed the 
decidua of pregnancy. They are intimately associated with and essential to the 
development of the maternal part of the placenta, and a more detailed and complete 
account of the decidua and of the modifications of its different parts is therefore 
necessary. 
The decidua is formed by the mucous membrane of the uterus, a hollow, thick- 
walled, muscular organ, situated in the cavity of the pelvis. The mucous membrane 
contains numerous tubular glands embedded in an interglandular tissue formed of 
round and irregular cells. The uterine glands are lined by cubical or columnar 
epithelium, and they open in the cavity of the uterus on a surface which is also 
covered by columnar cells. The whole of the mucous membrane is plentifully 
supplied with blood-vessels which pass into it from the surrounding muscular walls, 
and it is transformed into the decidua by proliferation and hypertrophy of all its 
parts. The interglandular tissue increases in amount and its blood-vessels dilate, 
especially near the surface of the membrane; but the most striking of the early 
changes occur in the glands—they become longer, more tortuous, their apertures 
enlarge and assume a funnel-like appearance, and they dilate a short distance from 
Allantoic 
diverticulum 
Primitive ali- 
mentary canal 
Yolk-sac 
Ectoderm 
Fetal villus™ ms Somatic mesoderm 
Yj 
psu 
Y 
Decidua vera ‘Splanchnic mesoderm 
‘Entoderm 
Decidua vera 
