632 
RICHARD ASSHETON. 
I ventured to suggest that this spot had nothing to do with 
archenteron formation, and that it might be a quite erroneous 
conclusion to identify it with the anterior end of the primitive 
streak of later times. I argued at some length to show from 
their description that their interpretation is not tenable. As 
an alternative I offered the suggestion that the spot in question 
is the morphological lower pole of the egg, and gave figures 
illustrating a comparison between Ornithorhynchus as de- 
scribed by Wilson and Hill and a Sauropsidan, such as the 
sparrow, postulating a complete growth round by the edge of 
the blastoderm at an earlier stage in the Prototherian than in 
the Sauropsidan egg in correlation with a smaller quantity of 
yolk. The fact that there is a very well-marked eccentricity — 
I mean that this spot is not diametrically opposite to the 
centre of the “embryo” in Ornithorhynchus as it is in the 
sparrow, but as it is not in the rabbit — did not seem to me to 
be a serious objection. 
Since making this criticism I have come across a series of 
sections which I cut through the lower pole of the egg of 
Tropidonotus natrix some years ago, but which I had 
forgotten — at a stage represented by the outline drawing 
(PI. 33, fig. 1). 
I find I have also in toto the same spot indicating the 
coalesced edges of the blastoderm of another egg of the same 
snake, and the appearances of these specimens are such as to 
add very considerably to the degree of probability that the 
suggestion I made will turn out to have been well founded. 
The resemblance between the drawings of sections taken 
through the centre of this area (fig. 2) and Wilson and Hill’s 
text-fig. 4 (p. 51) of their section through the archeuteric 
knot of Oniithorhynchus is most striking. It must be 
remembered that at this stage in the snake’s egg the “ blasto- 
cyst cavity” is still nearly filled with yolk, which is not 
shown in my drawing, as the loose yolk has been all washed 
off during the preparation of the specimen, this loose yolk 
corresponding to the “tolerably large remainder of the 
original yolk as a more or less coherent mass, lying free within 
