98 PROFESSOR TURNER ON THE PLACENTATION OF THE SLOTHS. 
have exhibited, not a regular placenta, but a number of distinct cotyledons. 
But, further, cases have been seen* in which the human placenta was sub- 
divided into two not quite equal parts, a condition which is normal in the 
tailed monkeys of the old world, and an approximation to which, as I have 
already stated, is seen in the placenta of the sloth. Some recent observations 
by ReicHERTt on a young human embryo at the 12th or 13th day, have 
shown that, at this very early period, distinct elevated islets or cotyledons 
had formed in the uterine mucosa, and a median cleft separated these islets 
into two halves, so as to give a bilaterally symmetrical arrangement. If, as is 
not improbable, these islets are the rudiments of the lobes of the future 
placenta, the human placenta approximates in its bilateral arrangement at this 
stage of development to what I have seen in the sloth. 
Both in the human and sloth’s placenta, curling arteries and utero-placental 
veins are present, though in the former their subdivision in the substance of the 
placenta into branches cannot be followed out as in the latter, in which animal, 
moreover, the dilatation of the veins into large sinuses within the muscular wall 
of the uterus and the serotina does not exist as in the human female. In both, 
intra-placental maternal sinuses, communicating on the one hand with the curling 
arteries, on the other with the utero-placental veins, are met with. In the sloth, 
however, these sinuses retain their individuality, their walls can be isolated from 
the adjacent villi, they have a tubular form, and their arrangement as an anasto- 
mosing network is preserved.{ The mode in which they wind in and out between 
the villi bears a strong resemblance to the description and figures given§ by 
Dr PrisstLey of the arrangement of the maternal vessels in a human embryo 
at the second month ; only in his case the vessels, though described as “capacious 
capillaries,” were not dilated into large sinuses as in the sloth. In the fully- 
formed human placenta, on the other hand, though the walls of the curling 
arteries and utero-placental veins are distinct structures, yet the intra-placental 
maternal blood sinuses are not tubular, but consist of a system of irregularly- 
formed and freely-communicating spaces. Whether they possess delicate 
walls separating them from the tissue of the foetal villi, or whether the villi 
float naked in the mother’s blood, are questions which have been much debated 
amongst anatomists. Every one who has examined the villi of the human 
chorion is familiar with the layer of nucleated cells which invests the villi like 
acap. It has been repeatedly figured, and is seen to lie immediately outside 
the single or double capillary loop which the bud-like offshoots of the human 
villi contain. Opinions are divided whether this layer of cells belongs to the 
* See Hecker, as quoted by Dr J. Matrnews Duncan in “Edinburgh Medical Journal,” Nov. 1873. 
+ Reicuert und pu Bors Reymonn’s Archiv. p. 127, 1873. 
+ The arrangement in the sloth is, indeed, not unlike what E. H. Weper conceived to be the 
arrangement in the human placenta. 
§ Lectures on the Development of the Gravid Uterus, p. 62, figs. 19, 20. London, 1860. 

