86 PROFESSOR TURNER ON THE PLACENTATION OF THE SLOTHS. 
whilst it is absent, he says, in the embryos of Dasypus, Calogenys, Dasy- 
procta, Hydro-cherus, Cervus, Ovis, Bos, Didelphis, Ursus, and Felis. But 
he describes it in Cwlogenys paca, Cervus, and the human embryo, as an epitri- 
choid layer, consisting of cells of smaller size than those which constitute 
the epitrichium proper in the animals which possess that membrane. This 
epitrichoid layer of cells probably takes a part in the formation of the vernix 
caseosa. He regards the epitrichium as no otherwise than the most superficial 
layer of the epidermoid-plate (Epidermoidal-Blattes) of the embryo. He believes 
that the outermost layer of the embryo (Horn-Blatt) divides into two parts, the 
deeper of which is the epidermis, properly so called, consisting both of its 
Malpighian and horny strata, whilst the superficial part is the epitrichium, 
which originally is blended with the epidermis, and only becomes elevated 
from it as a distinct membrane in consequence of the development of the hairs. 
My examination of the structure and arrangement of the corresponding 
membrane in Cholwpus Hoffmanni substantiates, in every important particular, 
WELCKER’S description, and leads me to form the same conclusion of its 
morphology as that to which he had previously arrived. More especially I may 
refer to its continuity at the oral, anal, and nasal orifices with the mucous lining 
of those passages, to its union also with the epidermis at the roots of the nails, 
and to the fact that it is perforated by the stronger tactile hairs, as evidences of 
its development from the Horn-Blatt. Moreover, I have been able to point out, 
what WELCKER and the previous observers were not, owing to the absence of 
the placenta in their specimens, in a position to ascertain, that a simple bag-like 
amnion exists in the sloth as in other mammals. The Epitrichium, therefore, is 
not the amnion, but is derived from the epidermoid layer of the embryo, 
the most superficial stratum of which is not perforated by the hairs (except the 
tactile bristles) in their development, but is elevated in the form of a distinct 
continuous membrane.* 
Some observations by G. Smmon on the development of the hairs in the pig 
(op. cit. p. 370), and of WELCKER on their development in Bradypus tridactylus 
(op. cit. p. 14), would seem to throw some light on the mode of production of 
the epitrichium. In both these animals the hairs do not lie straight in their 
follicles and with their apices directed outwards, but the individual hairs are 
coiled on themselves, or bent in the form of a loop in their respective follicles, 
so that the apex of each hair is in proximity to the root. It is not improbable 
* Through the kindness of my friend, Dr Joun Youne, Professor of Natural History in the 
University of Glasgow, I was enabled, in the month of July, to examine a foetus of Bradypus tridactylus 
in the Hunterian Museum in that University, though not one of Dr Wittiam Hunter's specimens. 
The foetus was 91 inches long from nose to tip of tail. It was covered with hair, and possessed a 
complete epitrichium similar to the larger of the two specimens figured by WeELcKER in his Memoir. 
The placenta, though greatly shrivelled. and hardened by the prolonged action of spirit, was seen 
to be composed of a number of discoid lobes. 
