PROFESSOR TURNER ON THE PLACENTATION OF THE SLOTHS. 99 
villus, or is a layer of cells derived from the decidua serotina, which has become 
blended with the tissue of the villus; but whether we regard these cells as proper 
to the villus or as belonging to the serotina, no corresponding layer was seen 
in the sloth. In the course of observations made during the past two years on 
the minute structure of the human chorionic villi, I have more than once seen 
an appearance which led me to believe that outside this layer of well-defined 
cells—.e., nearer, or rather next to the maternal blood—was a layer of squamous 
cells, which would represent, therefore, the endothelium of the maternal blood- 
vessels, blended with the villus owing to the great expansion of these vessels 
into the irregularly-shaped, freely-communicating blood sinuses. In the sloth, 
again, as the intra-placental sinuses retain the individuality of their walls, their 
endothelium remains in its proper position as a layer of cells lining the maternal 
blood-tubes. In the sloth, therefore, the capillaries of the chorionic villi lie 
closer to the periphery of the villus than is the case in the human placenta ; 
but, further, in the sloth these capillaries are arranged as a distinct network, 
whilst in man they form single or double loops. The branches which arise from 
the stems of the villi in the sloth are more elongated, and have more of a lami- 
nated arrangement than is the case in the human placenta. 
The presence of a system of maternal sinuses within the placenta of the 
sloth, though, as I have just pointed out, it differs in several points from the 
corresponding arrangement in man, is of great anatomical interest; for it 
presents a transitional form between the simple maternal capillary plexus met 
with in a diffused cotyledonary or zonary placenta, and the irregularly-formed 
blood-spaces which exist in the placenta of the human subject. Several obstet- 
rical writers have, from time to time, denied the existence of intra-placental 
maternal blood sinuses in the human female, and the objection has been 
advanced that the presence of such sinuses receives no support from compara- 
tive anatomy. In a communication read to this Society, May 20, 1872,* 
I adduced a number of facts, derived from the study of the human placenta, 
in support of the Hunterian doctrine of the intra-placental circulation of 
maternal blood, and I may now add to these facts the confirmatory evidence 
afforded by the structure of the placenta in the sloth. 
The condition of the utricular glands in the mucosa of the human and sloth’s 
uterus is also a feature of much interest. In both, in the quiescent, non-gravid 
state, the glands are short, comparatively simple in form, and are not demon- 
strable with the same readiness as in the uteri of animals, which, in the gravid 
state, possess a diffused or a zonary placenta. In the human uterus, at the 
commencement of gestation, the glands are well marked, but as pregnancy 
advances to its middle period they disappear, so that no traces can be seen of 
* Abstract in Proceedings of that date, and more fully in Journal of Anatomy and Physiology, 
November 1872. 
VOL. XXVII. PART I. 2C 

