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THE PLACENTA. 5d 
vascular, and it diminishes very considerably in thickness. The epithelium dis- 
appears from the spaces in the spongy layer, and the spaces themselves are 
flattened out into long sht-like clefts, in which condition they remain till 
the period of pregnancy is completed. The decidua vera is thus reduced to the 
condition of a relatively thin membrane, and its inner surface is fused with 
the remains of the decidua capsularis. 
The Decidua Basalis.—Apart from the changes due to the invasion of the 
foetal villi the most important transformations in this part of the decidua occur in 
the stratum compactum. The alterations in the spongy layer are similar to those 
which occur in the same layer of the decidua vera, viz. the lining epithelium dis- 
appears and the spaces are flattened out into a layer of cleft-like slits. 
In the stratum compactum, however, much more striking changes occur; the 
superficial epithelium and the tubular portions of the glands disappear, but the 
blood-vessels become greatly dilated, and, consequently, the layer increases consider- 
ably in thickness. The terminal loops of the small blood-vessels which le in the 
superficial part become converted into enormous blood-sinuses, but in the deeper 
part of the stratum a thin layer, which lies next the stratum spongiosum, remains 
relatively unchanged; this deeper part is called the basal layer, and through it 
the blood-vessels pass to and from the blood-sinuses in the more superficial 
portion of the membrane. When it is completed, therefore, the maternal portion of 
the placenta, which is the transformed decidua basalis, no longer consists of the 
stratum compactum, the stratum spongiosum, and the unchanged ay er, but it is 
formed from within outwards of—(1) a layer of blood-sinuses, (2) the basal layer, (5) 
the modified spongy layer, and (4) the unchanged layer. The difference between the 
decidua basalis and the maternal part of the placenta may be tabulated as follows :— 
Decidua basalis. Maternal placenta. 
{ Layer of blood-sinuses. 
\. Basal layer. 
Stratum spongiosum : Modified stratum spongiosum. 
Unchanged layer. : ; ; Unchanged layer. 
Stratum compactum 
It must not be forgotten, however, that whilst the changes which result in the 
formation of the maternal placenta out of the decidua serotina are taking place the 
stratum compactum has been invaded by the placental villi. 
The first result of this invasion is the destruction of the superficial epithelium 
of the decidua, which entirely disappears wherever the ectoderm of the feetal villi 
comes In contact with it. Afterwards the ectoderm of the villi, always in advance 
of the main body, reaches and surrounds the dilated decidual vessels, destroys the 
intervening tissues, and ultimately replaces the endothelial walls of the vessels, 
which by this time have dilated into enormous spaces. Into these spaces the 
ramifications of the villi project, and, as the endothelial walls are destroyed, they 
lie directly within the cavities of the spaces, and are surrounded on all sides by 
maternal blood. The most peculiar feature of this part of the placenta, when 
fully developed, is that the whole of the maternal portion of it, except the blood, 
has been removed and replaced by fcetal tissues, so that, although the maternal 
blood continues to circulate in the same spaces which it has occupied from the 
first, viz. the blood-sinuses in the superficial part of the stratum compactum of the 
maternal decidua, yet the walls of these spaces have been replaced more or less 
completely by foetal ectoderm, and, consequently, the spaces now lie in the midst of 
the foetal tissues. 
The invasion of the maternal by the fatal part of the placenta proceeds as far 
as the basal layer, and in this region the foetal ectoderm is directly continuous with 
the walls of the maternal blood-vessels at the points where they enter the sinuses. 
Although the invasion of the decidua basalis is so complete, some portions of 
the maternal tissues persist; thus the basal layer and many strands of the stratum 
compactum escape destruction. The latter extend from the basal layer to the 
outer surface of the chorion, and they are eventually converted imto fibrous strands, 
which divide the superficial part of the completed placenta into lobular areas. 
The completed placenta consists therefore of closely intermingled and fused 
