256 
RICHAKl) ASSHETON. 
egg ; reptilian because the neurenteric canal (metenteron) 
is so much more marked in the later stages than in any bird 
or even any other mammal, and avine because of the far 
greater rapidity with which the subgerminal cavity forms 
and swells up by infiltration of fluid and the great lengthening 
of the primitive streak, which is so characteristic of all birds 
and is so little marked in reptiles, though fairly well mai-ked 
in some Eutherian mammals, e. g. rabbit, carnivora, ungulates. 
My reasons for suggesting the above are that I cannot 
quite follow Wilson and Hill in their identification of the 
“ embryonic region ” of their fig. 5, Plate 2, with the structure 
in question, called “primitive knot,” fig. 11, Plate 5, or of 
this primitive knot with the neurenteric canal or protochordal 
wedge development of the later stages, their archenteric 
knot of their text-fig. 10 on p. 73. 
Very considerable gaps in their material occur between 
these three stages. 
The earliest of the three stages, a, showing the primitive 
plate, is a blastocyst measuring 6 x 6’5 mm. 
The middle stage, Q, was found in four eggs which measure 
10 ram. X 9‘5 or 9 mm. 
The last of the three, P, measures 17’5 mm. 
Comparing the three stages with corresponding ones of a 
rabbit : 
1st, a = 120 hours. 
2nd, Q = 168 hours. 
3rd, P = 188 hours. 
As regards the first the comparison is difficult, because I 
cannot see that there is any real ground for considering that 
any layer present in the platypus is the homologue of the 
troplioblast, unless — which is not at all improbable — the mass 
of yolk and cells forming part of the so-called “primitive or 
archenteric knot” (= germinal wall ?) is the mass of yolk- 
cells into which the epiblast in the Eutherian mammal’s egg 
has sunk, and by which it becomes temporarily covei’ed. 
But the second stage, i.e. Specimen Q,., fig. 7, is very 
much like the Eutherian mammalian embryo of the type which 
