266 
RrCIIAlfl) ASSHETON. 
meroblastic egg, because certain things point to the proba- 
bility of the type of Eutherian development such as we see 
in Insectivora, Cheiroptera and Primates (Anthropoidea), 
with cumulate placentation, being primitive as regards 
Euther ia. 
If we accept Hubrecht’s hypothesis that the types just 
mentioned are primitive as regards the Eutheria, Ave may 
nevertheless derive this from the large-yolked egg on the 
supposition that the trophoblast represents the reduced yolk- 
mass which has temporarily overflowed the inert epiblast, and 
yet retain the meroblastic type of egg for its pre-mammalian 
ancestry. 
The only difficulty to which this leads one is that the 
amnion, such as we find it in Eutherian types like rabbit or 
sheep or pig, is not strictly homologous to the amnion of the 
Sauropsida. That an amnion so like as that of a sheep and 
that of a bird should be not absolutely homologous may seem 
to some improbable. Jenkinson (’00) objected to this view 
on a previous occasion. To whom I Avould answer thus — 
In one sense all the amnions of the Amniota are homo- 
logous. There can have been no discontinuity in the presence 
of an amnion. The one kind cannot have ceased to exist 
until the other had come into existence. It is not a case of 
homoplasy only. But I submit that the amnion of the rabbit 
is not more homologous to the amnion of the sauropsidan 
than the horny teeth of Ornithorhynchus are homologous to 
the true teeth of the mammal or reptile, Avhich they have 
supplanted. 
A similar difficulty is with us already, within the small 
group of Eutherian mammals, and must be faced ; for the 
amnion of the Eutherians, like the sheep, can hardly be 
strictly homologous to the amnion of Cavia, because whereas 
the former is formed partly of cells of the epiblast plate of 
the embryonal area and partly of trophoblast, the latter is 
formed wholly of cells of the embryonal epiblast. 
In view of the fact of the curving of the epiblastic plate 
noticed in so many cases — Sus, Tupaja, Talpa, Vespertilio, 
