THE PLACENTA. 51 
THE PLACENTA. 
The placenta is the organ of foetal nutrition and respiration. 
In it the blood-vessels of the foetus and those of the uterus are brought into 
such close relationship with one another that free interchanges readily take place 
between the blood of the 
mother and that of the 
foetus. In the simplest 
form of a placenta the 
foetal villi are merely em- 
bedded in the maternal 
mucous membrane, and 
the relationship between 
foetal and maternal blood 
is not very close. In other 
forms, eg. the human 
placenta, the relation of 
foetal to maternal blood 
is much more intimate; 
this involves marked modi- 
fications in the elements 
of the placenta, and its 
structure becomes corre- 
spondingly more complex. 
In all forms, however, the 
placenta consists of foetal 
and maternal portions. 
Before the unpregnated 
ovum reaches the uterine 
Decidua basalis 
( cmaetaae Unchanged ayer tani 
Gland/ Funnel- y, Stratum spongiosum mucosa 
| shaped Stratum compactum aot 
mouth — Blood-vessel 
which ; 
becomes Ectodermal villus 
ecelom 
Decidua 7~—Inner cell mass 
vera 
(Entoderm) 
Cavity of uterus. Decidua vera 
Fic. 37.—Dracram, showing the relation of the young human ovum to 
the decidua. 
The ectoderm is distinct from the inner cell mass, but as yet there is no 
entodermal cavity in the latter. 
cavity the mucous membrane of the uterus undergoes 
(=) 
certain changes in preparation for its reception and retention, and the modified 
mucous membrane is 
Decidua basalis 
Unchanged layer 
Stratum spongiosum 
Stratum compactum ~ 
Pz 
known as the uterine 
| Maternal vessel 5 
decidua. These changes 
We tee oe are, for the most part, 
_Mesoderm hypertrophic; the  vas- 
Placental villus a 
Cavity of — 
blastoderm 
Cavity which 
becomes ccelom 
Decidua vera 
Fic. 38.—Diacram, showing a further stage of development of the human 
cularity of the mucous 
membrane is increased 
mainly by the dilatation 
of its veins and capillaries, 
the tubular uterine glands 
become elongated, ir- 
recular, and tortuous, and 
they dilate both at their 
orifices and in the deeper 
; Ss part of the mucous mem- 
Entoderm brane; at the same time 
Peli Vee the interglandular con- 
nective tissue proliferates, 
and as a result thedecidua 
is thicker,softer,and more 
ovum and its relation to the decidual tissues. The entodermal cavity Spongy than the unaltered 
or yolk-sac has appeared in the inner cell mass, and the mesoderm ynucous membrane. 
has commenced to extend from the primitive streak in two layers, 
splanchnic on the yolk-sac and somatic on the ectoderm. 
it merely rests, at first, 
When the developing 
ovum enters the uterus 
on the surface of the decidua, but by the growth 
of a circular fold of the decidua it becomes gradually covered, and on the fusion 
of the margins of the fold it is entirely enveloped. As soon as the circular fold 
which surrounds the ovum arises it is possible to distinguish three decidual areas : 
