78 GENERAL ANATOMICAL CHARACTERS 
the vessels which, with their surrounding membranes, consti- 
tute the umbilical cord—the medium of communication between 
the foetus and the placenta, when that organ is fully de- 
veloped. 
The ege membranes of the Monotremes present many points of 
agreement with those of the ovum of the Marsupials,! and differ 
from those of the Placental types. Thus Monotremes and Marsu- 
pials agree in having a vitelline membrane, which appears between 
the young ovum and the follicular epithelium, persisting in the 
one case until the time of hatching, and in the other till a late 
uterine stage. There are also several other common features fully 
described in Mr. Caldwell’s memoir, but which cannot be detailed 
in this work. 
In the Marsupialia the observations made many years ago by 
Sir R. Owen upon the development of the Kangaroo have been 
confirmed by those of Dr. H. C. Chapman,? while Dr. Selenka,? and 
Professor H. F. Osborn have contributed important evidence as to the 
structure and relations of the foetal membranes of the Opossums 
and others. It thus appears that up to the period of the very 
premature birth of these animals the outer covering of the ovum, 
or false chorion, is free from persistent villi, and not adherent 
to the epithelium of the uterine walls; for, although fitting into 
the folds of the latter, it is perfectly and readily separable in its 
entire extent from them. The umbilical vesicle or yolk-sac is large, 
vascular, and adherent to a considerable portion of the false chorion 
or subzonal membrane, while the allantois is relatively small, and 
although the usual blood-vessels can be traced into it, it does not 
appear to contract any connection with the false chorion, and, there- 
fore, much less with the walls of the uterus, of such a nature as to 
constitute a placenta. In some forms, however, such as the 
Opossums, the umbilical vesicle or yolk-sac develops temporary 
villi, which unite with the subzonal membrane, or false chorion, to 
form a disc-like area closely attached to the cells covering the 
utricular glands of the uterine epithelium, and thus forming a 
so-called yolk-sac placenta. The function of this organ is considered 
to be the transmission of the secretions of the utricular glands to 
the embryo by means of the umbilical vesicle ; the function of the 
allantois being [either respiratory or the absorption of the fluid 
secreted in the uterine cavity by the utricular glands. 
While in the uterus the nourishment of the fcetus seems, there- 
fore, to be derived from the umbilical vesicle, as in reptiles and 
1 See B. H. Caldwell—‘‘ The Embryology of Monotremata and Marsupialia,” 
Phil. Trans. for 1887, p. 463. 
2 Proc. Acad. Nat. Sct. Philadelphia, 1881, p. 468. 
° © Studien uéber Entwickelungeschichte der Thierc,” pt. 4, Wiesbaden, 1886. 
4 Journal of Morphology, vol. i. p. 873 (1887). 
