234 
lilCHARD ASSHETOX. 
usual plug of fine yolk (characteristic of the Avine egg as the 
nucleus of Pander) under the segmented area if the Monotreine 
egg is to he regarded as a trophoblastic vesicle, including 
“ besides an embi’jonic knob a very considerable amount of 
food yolk, the development of which will have gone parallel 
with the change in the ancestral line from viviparity to ovi- 
piirity.” 
There is no trace of a breaking through the trophoblast by 
the inner cells, which, on the contrary, seem to spread out 
under the outer layer into a thin membrane. When at a later 
period the primitive streak proliferating area seems to project 
through (Wilson and Hill, PI. 3, fig. 26) after the manner of 
Selenka’s figure of the mesoblast pushing through the epi- 
bhist in fig. 9, Taf. xviii, of the opossum, this condition could 
not be taken as the pushing of epiblast through a trophoblast, 
as the neural plate is undoubtedly in front of this area, and 
has been formed from the originally outer layer. 
So that neither in the Prototheria or the Metatheria is 
there any really tangible evidence of a trophoblast occurring 
as a covering layer over the definitive epiblast as in Eutheria. 
Summarising up to the present stage I submit that 
Hubrecht, while ignoring alternative interpretations, has not 
Tuade good his own case either for the presence of the tropho- 
blast layer in Prototheria and Metatheria, or for the origin of 
the trophoblast in Eutheria, as a special Embryonalhulle 
formed by delamination from the epiblast, and has not 
attempted to meet any of the objections presented to his 
theory by the study of the segmenting- stage of such mamma- 
lian eggs as those of Lepus, Ovis, and Vespertilio. 
Phylogenetic Origin of Trophoblast. 
Confirmed in an opinion which as regards Prototheria and 
l\Ietatheria is based on very doubtful evidence, that all classes 
of mammals have a larval envelope, the trophoblast, Hubrecht 
proceeds to speculate upon its origin outside the group, and 
starting with a ccfilenterate ancestor, says: "A tendency to 
