634 
lUCHARD ASSHETON. 
The part which pi’ojects above the surface, the actual cell- 
plug, is probably partly due to a growth of the epiblast cells 
after the coalescence of the margin, as I have found certainly 
one mitotic figure within this mass. In some sections, e. g. 
fig. 4, the distinction into “ cell-plug ” (c. j>.) and “ central 
more cellular zone ” (c. z.) is still more marked than it is in 
fig. 2. 
The part called by Wilson and Hill the “marginal or 
cortical zone of the knot tissue ” (m. z.) is represented by the 
germinal wall part of the hypoblast of Tropidonotus, which, 
like the author^s marginal zone, can be well described as 
showing “ coarsely reticular, indefinite and feebly staining 
characters,” and being “ poor in nuclei which are chiefly met 
with near its entodermal aspect.” 
Fig. 4 is another section of the same specimen which passes 
through a little cleft still remaining, which shows the relations 
of the thin epiblast to the thickened rim very clearly. 
It is difficult to resist the conclusion that a condition 
similar to this has given rise to the state of affairs in Orni- 
thorhynchus, described by Wilson and Hill and indicated in 
their text-fig. 5, p. 52, and figs. 9 and 10, pi. 4. 
If my comparison is a correct one, the archenteric knot of 
Ornithorhynchus with its anterior and posterior lips of the 
blastopore and its “commencement of true archenteric in- 
vagination ” may be dismissed, and another stumbling-block 
will be removed from the path of the student of mammalian 
embryology. 
As regards the question of the eccentricity of this spot in 
Ornithorhynchus I have no further evidence to bring forward. 
I have no note on this point and cannot recollect whether 
tlie point of coalescence of the blastoderm is diametrically 
opposite to the mid-dorsal surface or not. The egg of Tropi- 
donotus is long. Even if the point of coalescence in the 
snake’s egg is exactly opposite to the upper pole I do not think 
that it invalidates in any way the argument from the morpho- 
logical point of view. 
Whether, if this is the correct interpi’etation, it supports 
