PROFESSOR TURNER ON THE PLACENTATION OF THE SLOTHS. 83 
which were so arranged as to have the greater part, or sometimes the whole, of 
the circumference of their walls in contact with the highly vascular surfaces 
of adjacent villi (fig. 5 and 6). By this arrangement the foetal and maternal 
blood-streams were brought so close to each other that the interchange of 
material which without doubt takes place between them could be readily 
effected. The transverse diameter of the terminal villi was, as a rule, equal to 
that of the maternal blood sinuses lying next them, so that the placental lobe 
consisted of foetal and maternal structures in almost equal proportions. The 
average diameter of the foetal capillaries was about :0005 inch, or about j¢th 
of the diameter of the largest maternal blood sinuses. 
The terminal branches of the villi reached as far as the decidual surface of 
the placenta, and gave off lateral offshots, which were interposed between the 
decidua and those intra-placental sinuses which lay nearest it ; for foetal capil- 
laries filled with blue imjection were seen in this region sometimes cut across 
transversely, at others with their long axis parallel and internal to those 
maternal blood sinuses that came close up to the uterine face of the placenta. 
The basis substance of the villi consisted of delicate connective tissue in 
which the corpuscular element was strongly pronounced, not only because the 
corpuscles were numerous in a given area, but from their size. In shape they 
were by no means uniformly spindle-like, but many of the cells were irregular 
or broadly ovoid or even circular. They consisted of dimly granular protoplasm, 
and were distinctly nucleated (fig. 5 and 6). I examined a number of villi 
dissected out of the substance of the placenta with reference to the existence 
of an epithelial layer on the free surface outside the capillary network, but 
without obtaining any evidence of its presence. Neither could I detect any 
evidence of a layer of cells investing the villi similar to the well-known layer 
which forms a cap for the villi of the human chorion, and which has by 
some anatomists been referred to the decidua serotina. In dissected prepa- 
rations, where the villi and maternal blood-vessels had been torn asunder, I 
sometimes saw flakes of delicate fibrillated tissue attached to the periphery 
of a villus, which I believe to be portions of the wall of the maternal vessel 
torn off during the dissection. I looked carefully for evidence of the prolonga- 
tion into the placental lobes of processes or bars of the decidua serotina, but 
without seeing any, though, as has previously been stated, the uterine face of 
the placenta was invested by this membrane. 
At the edges of those placental lobes, which were separated from each other 
by intermediate narrow strips of chorion, the uterine surface of that membrane 
gave origin to elongated and branched villi. The blue injection had entered 
the vascular trunks within the stems of the villi, but no capillaries were injected. 
The uterine mucosa was in contact with the free ends of the villi, and even in 
part gave off processes which passed between them, but no maternal blood 
VOL. XXVII. PART I. XG 
