584 
PROF. W. TURNER ON THE PLACENTATION OF THE LEMURS. 
In constructing his pedigree of the Mammalia, Haeckel, though admitting the difficulty 
of solving the question, is inclined to think that the placental mammals had at once 
diverged into two completely distinct groups, a Non-deciduate and a Deciduate. Out of 
the Non-deciduata, he says, had proceeded the TJngulata and Cetacea , with their diffused 
or cotyledonary forms of placenta ; out of the Deciduata had arisen the Insectivora, 
Carnivora , Chirojptera, Iiodentia, Hyracoidea, Proboscidea, Lemurs, Apes, and Man, with 
a zonary or discoid form of placenta ; whilst the primaeval form common to all the 
Deciduata was the group of Half- Apes, or Prosimice, of which the Lemurs of the present 
day are the descendants and representatives. 
The principle which guided Haeckel in framing his pedigree of the Mammalia ren- 
dered it necessary for him to assume that the Lemurs possessed a deciduate placenta, so 
that they might serve as the primary group of origin of the deciduate mammals. But 
I have just shown that in these animals the placenta is diffused and non-deciduate, so 
that the Lemurs must be transferred to the Non-deciduata, and, on the hypothesis 
advocated by Haeckel of an independent origin of the Deciduata and Non-deciduata, 
they can no longer be regarded as the root form of the deciduate mammals. 
It appears to me, however, that the conditions of the theory of descent may, so far as 
it is based on a consideration of placental characters, be more satisfactorily provided for 
by assuming that the deciduate placenta had been evolved from the non-deciduate, rather 
than that an abrupt divergence into two distinct placental groups had occurred*. No 
one, I suppose, would doubt that the diffused placenta has the most simple mode of 
structure, and that the distribution of the villi over the surface of the chorion presents 
a close approximation to the primary embryonic arrangement ; whilst the discoid 
placenta exhibits the greatest departure from the diffused villous chorion of the early 
embryo. Not only is the placenta in the discoid form concentrated in a limited area, 
but its internal structure is much more complicated ; for the diminution in the relative 
proportion of the surface of the chorion, subserving the function of a placenta, has ren- 
dered necessary a greater degree of complexity in the form of the villi and in the foldings 
of the uterine mucosa throughout the placental area. It seems to me, therefore, to be 
more in accordance with a theory of descent that the complex form of placenta should 
be regarded as having been evolved out of the simple, than that each should have arisen 
independently, as is assumed by Haeckel, out of a non-placental subclass of Mammalia. 
I have elsewhere recorded f observations on the structure of the placenta which show 
that the line of demarcation between the Non-deciduata and Deciduata is not so abrupt 
as has usually been supposed, but is graded over by an intermediate arrangement — the 
passage from the diffused placenta, in which no maternal tissue deciduates during par- 
turition, to those deciduate placentae in which both the epithelial and subepithelial 
* In the last of my Lectures on the Comparative Anatomy of the Placenta, delivered at the Royal College 
of Surgeons, June 1876, and printed in the Journal of Anatomy and Physiology, Oct. 1876, I have discussed 
at much greater length the structure of the placenta in relation to the theory of evolution. 
f Proceedings of the Royal Society of Edinburgh, 1875. 
