82 PROFESSOR TURNER ON THE PLACENTATION OF THE SLOTHS. 
in which they exist shows that the nucleated character of the red blood- 
corpuscle in this mammal is an exceptional occurrence. 
I have up to this time spoken of the serpentine and convoluted microscopic 
blood-tubes as “ vessels,” without stating whether I look upon them as arteries, 
as capillaries, oras veins. In the great delicacy of their coats, and in the absence © 
both of muscular and elastic tissue, they approximate to capillaries, but they differ 
from them again in their large size and dilated character, so that they might be 
spoken of as forming a system of intra-placental maternal sinuses, continuous 
on the one hand with the curling arteries, on the other with the utero-placental 
veins, through which the maternal blood flows in order to be brought into 
relation with the capillaries of the foetal villi Of maternal vessels pos- 
sessing the usual calibre of capillaries I saw not a trace, so that I am of 
opinion that these serpentine and convoluted vessels, or sinuses, are the only 
channels of communication within the placenta between the uterine arteries and 
veins. That these sinuses are neither artificially produced by the injection, nor 
a system of wall-less cavernous spaces, but a definite arrangement of blood- 
channels, is proved by their continuity with the uterine vessels, by the posses- 
sion of a distinct wall capable of isolation, and by the enormous quantities of 
blood-corpuscles which they contained. 
I carefully examined the decidua serotina im sections where the placenta 
was still 27 situ, as well as in fragments cut off after the lobes were peeled from 
the surface of the uterus, but could not detect any evidence of the presence of 
utricular glands; so that these structures appear to be absent both from the 
placental and non-placental areas of mucous membrane. 
My attention was now directed to the arrangement and structure of the villi 
of the chorion. The stems of the villi arose at intervals from the surface of the 
chorion, and extended almost vertically through the thickness of the placental 
lobes close up to, or even as far as the decidual surface of the organ (fig. 4). In 
their course they diminished in thickness and gave off branches, which extended 
obliquely, aud for a considerable distance, away from the main stem; these 
branches in their turn gave origin to smaller branches, from which again short 
bud-like sprouts proceeded, giving to the entire arrangement a characteristic 
tree-like character. Closely adapted to the sinuous outlines of the branching 
villi were the sinus-like maternal blood-vessels. Well-marked branches of the 
umbilical vessels coursed along the axes of the stems of the villi, and gave rise 
to smaller vessels, which extended down the branches of the villi, and ultimately 
jomed a well-marked capillary plexus, which was distributed close to the sur- 
face, not only of the branches of the villiand their bud-like offshoots, but of the 
stem of the parent villus. In those parts of the placenta where the umbilical 
vessels were well injected these capillaries were filled with the blue gelatine, 
and were seen to lie in close relation to the tortuous maternal blood sinuses, 

