1886. | Embryology. 179 
Mammais.—Mr. Sidebotham (Proc. Zodl. Soc. London, 1885) 
gives a detailed account of the myology of the water opossum, 
Chironectes variegatus. The discovery of the wild cat (Fesis 
catus) in Ireland, is often reported, but investigation has always 
shown that the supposed wild cat was but a feral specimen of the 
domestic cat. A leopard skin in which most of the rosettes are 
replaced by black spots, numerous and of small size, has been 
brought from South Africa, and is the first African species which 
exhibits the tendency to melanism so strongly developed in some 
Asiatic individuals. Mr. O. Thomas (P. Z. S. 1885, 329), dis- 
tinguishes three varieties of the echidna, viz: Æ. lawesi, aculeata 
and sefosa. The only remaining recent species of the family is 
Taglossa bruijni, a larger animal, found in Northwestern New 
uinea new species of paca (Calogenys taczonowskt) is de- 
scribed by Sulzmann, who obtained it in Western Ecuador, where 
it inhabits mountains between 6000 and 10,000 feet above the sea. 
Like the well-known paca, it digs a burrow with two openings. 
The native name is Sacha-cui. 
EMBRYOLOGY .* 
_ THE ORIGIN oF THE AmNion.—The purpose of the present note 
is to point out some of the mechanical conditions and causes 
which have been competent, in the course of the development of 
development, to bring about the formation of the amnion. 
embryological writer, as far as I am aware, has ever attempted to 
trace the amnion to the part in the embryos of anamniated forms 
which led up to its development in the amniated ones. Balfour 
said, that “it does not seem possible to derive it from any pre- 
existing organ” (Comp. Embryol., 11, 256). And he says further 
(op. cit., 257): “The main difficulty is the early development of 
the head-fold of the amnion.” Balfour's view, that it is developed ` 
part passu with the outgrowth of the allantois, is utterly inade- 
quate to explain the genesis of the amnion of insects or that of 
Peripatus edwardsii and P. torquatus, for in them no allantois is 
formed. His hypothesis also breaks down in the light of the 
eae researches of Selenka on the inversion of the layers in 
e a. 
_ A comparison of the longitudinal, vertical, diagrammatic sec- 
tions, figures A and B, of an osseous fish-egg and a mammalian 
ovum respectively, will 
conclusively show that 
the somatopleure s, in 
A, is the exact homo- 
‘Edited by JoHN A. RYDER, Smithsonian Institution, Washington, D. C. 
