EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 19 
from the Didelphia onwards, where either the omphaloidean 
or the umbilical arteries, or both, serve that purpose. 
This then is my interpretation of the phylogenetic phases 
through which the trophoblast has passed. They cannot be 
said to be numerous or intricate, nor can the interpretation 
be looked upon as strained or artificial. _The less so, because 
in all mono- and di-delphic mammals, which have as yet been 
examined, we do—as was noticed above—encounter a larval 
envelope—the trophoblast—which surrounds the formative 
cells of the embryo. Without exception the trophoblast 
undergoes the series of changes and physiological trans- 
formations here sketched, becoming first vesicular, then 
selective to certain nutritive matter, finally vascularised and 
locally strongly adhesive to and fused with maternal tissue. 
B. OrRNITHODELPHIAN MAMMALS AND SAUROPSIDA. 
The segmented egg of Ornithorhynchus and Hchidna, the 
two living representatives of the Ornithodelphia presents 
itself to us under a totally different aspect, as compared to 
the other mammals. The ornithodelphian egg does not cleave 
according to the holoblastic, but to the meroblastic type, and 
offers numerous points of comparison with that of reptiles 
and birds. However, our knowledge of it is as yet very 
scanty and limited to what Caldwell (87), Semon (94), and 
Wilson and Hill (03, 707) have taught us. The egg is 
enclosed in a leathery shell. There is no, or hardly any, 
albumen, and this makes investigation of the earliest stages 
all the more difficult. 
The formative protoplasm, accumulated at the upper pole 
of the yolk, breaks up into a number of cleavage-cells (Fig. 
66) and at a very early stage the outer layer, already 
distinctly visible as such in the preceding stage (Fig. 67), 
has spread over the yolk as a membrane of flattened cells 
with flattened nuclei (Figs. 68 and 69). At the upper pole 
this layer covers—at the spot where the embryo is going 
