STUDIES IN MAMMALIAN EMBRYOLOGY. 387 
2. On the value of the primary division of the Monodelphia 
(Mammalia Placentalia) into Deciduata and Adeciduata. 
In the course of this memoir attention has repeatedly been 
called to the fact that the histological details of the placenta- 
tion of the hedghog, as acentral representative of a primitive 
order of monodelphous Mammalia, are far more intricate than 
was originally expected. 
The recent researches above referred to of Duval, Selenka, 
Frommel, van Beneden, Fleischmann, Strahl, Masius, and 
others, have shown that also in other orders of Mammalia 
monodelphia our acquaintance with the details of the process 
of placentation is far from being exhaustive. On the contrary 
their results foreshadow a period of increased activity, perhaps 
even of revolution, with respect to many questions belonging 
to this chapter of embryology. 
There is no doubt that the question of placental classification 
will have to be carefully reconsidered. 
On the two primary divisions of the Monodelphia which that 
classification has called forth, I wish to make the following 
remarks. 
Twenty-five years ago Huxley, in his admirable ‘ Lectures 
on the elements of Comparative Anatomy,’ proposed to sub- 
divide the monodelphous mammals into two groups which have 
since been known as Deciduata and Adeciduata. There can 
be no doubt that the placental characters of the mammals 
that are united in the latter sub-division have many particu- 
lars in common, and the fact that the majority of the genera 
thus linked together have on other anatomical grounds been 
already brought into juxtaposition under the name of the 
Ungulata, increases the probability that the particular “ non- 
deciduate ” placental characters are indeed the expression of 
genetical relationship and not of fortuitous similarity. The 
fact that certain though not all the genera of Edentata have 
to go amongst the Adeciduata, while other genera, as far 
as we are able to judge from the researches that have been 
hitherto published, agree in placental characters with the 
