
PLACENTATION OF THE SEALS. 287 
scales of nucleated protoplasm. The blood-vessels of the villi were derived 
from the umbilical vessels. The large trunks lay in the stems of the villi and 
branched in an arborescent manner, ending in a capillary plexus in the terminal 
villous tufts (fig. 8). In some of the larger branches a vessel ran parallel and 
close to the surface of the villus which communicated with the capillaries of the 
tufts arising directly from the sides of the branches. Those villi which entered 
into the formation of the greyish membrane conveyed the vessels which formed 
the capillary plexus situated in it. . 
The intra-lobular prolongations of the maternal mucosa did not pass directly 
from the non-deciduous layer of mucosa, investing the muscular coat of the 
uterus, into the lobules, for the greyish membrane situated on their uterine surface, 
and on the adjacent part of the sides of the lobule, prevented a direct entrance. 
The intra-lobular decidua was therefore derived from those processes of the 
decidua which dipped into the secondary and tertiary fissures. These processes, 
in the form of slender bands and lamine, penetrated up to the chorion, and 
then branched off laterally into the lobules where they formed the sides of the 
fissures, when they at once broke up into the reticulated lattice-like arrangement 
of sinuous trabeculze already described. In sections made through the lobules, 
where no displacement of the relative position of the foetal and maternal struc- 
tures had taken place, the meshes of the reticulum were seen to be occupied 
by the villous tufts, and not unfrequently the tufts were surrounded by a ring- 
like arrangement of trabecles. In this manner, throughout the entire lobule, 
the maternal and fcetal parts of the placenta were so closely intertwined that 
the two systems of blood-vessels were brought into close juxtaposition with 
each other: the structures which intervened being, on the maternal side, the 
epithelial investment of the trabecule, and on the foetal, the flattened scale-like 
superficial cells of the villi. The greyish membrane also contributed to the pro- 
duction of the juxtaposition, for not only were the folds on its deeper surface 
vascular, and projected into the lobule so as to have the maternal trabecule in 
contact with them, but it is not improbable that others of the capillaries which 
it contained were, when the placenta was in position, in relation with the 
capillary blood-vessels of the non-deciduous mucosa forming the irregular pits 
or trenches into which the placental convolutions fitted. In the non-deciduous 
as in the intra-lobular mucosa, a layer of epithelium covered the free surface 
of the maternal membrane. 
From the mode in which the placental lobules were walled in on the uterine 
aspect by the greyish membrane of fcetal tissue, from the processes of decidua 
having to penetrate up to the chorion before their capillaries entered the lobules, 
and from the recurrent course which so large a proportion of the intra-lobular 
trabeculee had to take in order to reach the villi situated nearest to the greyish 
membrane ; the maternal blood-vascular system penetrated throughout the entire 
VOL, XXVII. PART III. 4F 
