73 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 egg membranes of the Monotremes present many points of 

 agreement with those of the ovum of the Marsupials, 1 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, 2 while Dr. Selenka, 3 and 

 Professor H. F. Osborn 4 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 Avails 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, Avhich 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 foetus 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. Sci. Philadelphia, 1881, p. 468. 



3 " Studienueber Entivickelungeschichtc dcr Thierc," pt. 4, Wiesbaden, 1886. 



4 Journal of Morphology, vol. i. p. 373 (1887). 



