EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 139 
The intra-uterine nutrition is no longer accompanied—as in 
the more primitive Perameles—by fixation of the blastocyst 
against the uterine wall, and there is only a very loose con- 
nection between vascular maternal folds of the mucosa and 
the vascularised surface of the umbilical vesicle. Moreover, 
this connection is only of a very short duration, parturition 
taking place after eight to fourteen days, and the peculiar 
specialised nutrition in the marsupium coming into play 
immediately after. Still the early blastocyst of the Opossum 
shows the spongeous proliferation of the trophoblast (Fig. 
134), of which we may certainly say that it can contribute 
towards the absorption and elaboration of fluid material con- 
tained in the uterine lumen. It does not reveal marked 
propensities towards direct phagocytical action. Selenka 
found its lacune (87) filled with liquid which it most prob- 
ably derived from the contents of the uterine glands that 
had found their way into the uterine lumen. 
Summarising what we find in the Didelphia we may say : 
(1) in the more primitive forms: a well-fixed blastocyst which 
is united by a proliferating trophoblast to the syncytium that 
arose out of the maternal uterine epithelium. ‘lhe blasto- 
cyst is nourished by the combined results of phagocytosis 
and of osmotic exchange between on the one hand an allantoic 
and an omphaloidean vascular network with, on the other hand, 
a maternal lacunar circulation in a syncytium of mixed deriva- 
tion, the embryonic parts of which are resorbed by the 
maternal after parturition ; (2) in the secondarily specialised 
forms: a blastocyst very loosely held between numerous and 
intricate maternal folds with which it enters into osmotic 
exchanges by means of an omphaloidean circulation on the 
faintly convex surface above the embryo without any villi 
corresponding to the maternal folds. Moreover, an early 
trophoblastic proliferation in which probably absorption of 
fluid material, taken from the uterine lumen, is of more 
importance than eventually additional phagocytotic phe- 
nomena. 
In all existent genera of Didelphia the early ontogenetic 
