1887] Embryology. 781 
_ the development of the zonary, diffuse, or cotyledonary placenta 
would be established. 
Just here it may be well to call attention to the fact that the 
bare poles of the chorion in the diffuse type of placenta, as seen 
inthe Mare and Dugong, show conclusively that we cannot draw 
asharp line of demarcation between the diffuse and zonary types 
of the placenta. It follows, without any doubt whatever, from 
what has been said in the preceding paragraph, that the so-called 
| ffuse type of placenta is merely a special case of the zonary, 
_ in which smaller polar areas have been left bare of villi because 
of more extensive contact of the chorion with the uterine mucosa. 
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The concentration or diminution of the placental area also 
pends, to a great extent, upon the degree to which the allan- 
tois is extended over the subzonal membrane; but the gradual 
restriction of the area covered by the allantois must have had 
some determining: cause, The influence which would be ade- 
quate to produce such a restriction could only be a physiological 
one, determined by the mechanical relations of the parts involved, 
and it is therefore almost certain that the extension or restriction 
4 of the allantoic area would be determined by the proportions of 
the surface of the chorion brought into contact with the vascular 
‘alls of the uterus, or by the restriction of the vascular and de- 
~ Ciduous area in the walls of the uterus itself, The last-mentioned 
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order that a number of foetuses could be nourished in each 
baa t one time, as in Rodentia. 
> e latter, when the foetuses are somewhat less than half 
