6 GENERAL EMBRYOLOGY. 
On 
foetal and maternal tissues, through which both the foetal and maternal blood 
streams circulate. It is well adapted on account of its peculiarities of structure to 
fulfil the nutritive and respiratory requirements of the embryo. The foetal blood- 
stream which 
flows through the 
placental vill and 
Stratum oe the maternal 
Beebe =—= = Sees oN Placental villus blood-stream in 
compact the placental 
sinuses are only 
separated from 
each other by two 
layers of foetal 
ectodermal — epi- 
thelium and a 
small amount of 
foetal mesoderm, 
—Umbiliceal the latter being 
oo practically re- 
duced to thesingle 
Amnion layer of. endo= 
thelial cells which 
form the walls of 
the foetal vessels. 
Through these 
| Peri- layers, by osmosis, 
cardium and possibly by se- 
cretion, materials 
eae are passed both 
Fused decidua cap. (TO motherste 
sularis and decidua embryo and from 
ie embryo tomother, 
the placenta serv- 
ing not only for 
purposes of nutri- 
tion and respira- 
tion but also as an 
Fic. 43.—Driacram. Later stage in the development of the placenta, showing the excretory Oreae 
relations of the fcetal villi to the placental sinuses, the fusion of the amnion Whilst the pla- 
with the inner surface of the chorion, and the thinning of the fused decidue ceenta is attaiming 
(capsularis and vera), its full develop- 
ment the amnion is expanding, and finally its outer surface fuses with the inner 
surface of the chorion, consequently, the innermost portion of the placenta is 
provided with a covering of amnion. 
The full-time placenta is a discoid mass about 20 to 25 inches (50 to 60 em.) in 
circumference and 1} in thickness at its centre; it is much thinner, however, 
at its margins, where it is continuous with the membranes formed by the fused 
chorion, decidua vera, and decidua capsularis. Its weight is about one pound, and 
it consists from within outwards of the following layers :-— 
Placenta 
Unchanged laye? 
Yolk-sae 
Allantoie diverticulum 
Fused mesoderm of 
amnion and chorion 
Amnion f Ectoderm. 
. : ’ |. Mesoderm. 
Fetal ; Allantois with foetal vessels Mecadern: 
{ Mesoderm. 
\. Ectoderm. 
| Chorion 
Layer of maternal blood sinuses and remains of the interglandular 
tissue of the stratum compactum. 
Basal layer. 
| Modified spongy layer. 
Unchanged layer. 
Maternal « 
