THE “ARCHENTEEIC KNOT ” OE OENITHOEHYNCHUS. 633 
the cavity of the vesicle of the Ornithorhynchus egg of 
10-12 mm, in diameter (p. 42). 
Fig’. 2 is the 239th section of the series of 440 sections 
passing through the spot, and so is nearly central. 
This shows that the lower pole of the egg of Tropidouotns, 
after the yolk has become completely enveloped by the edge 
of the blastoderm, consists of the following parts : On the 
outside there is the epiblast (ep.), a thin layer, except at the 
centre where its free edge has concentrated forming the 
dense mass of cells (c. p.). Beueath this there is, towards 
the periphery, the hypoblast [hy.), consisting of a reticulum 
containing many yolk-spherules {y. y.), and small nuclei on 
the surface next the epiblast. This hypoblast layer is seen 
to be much thicker near the centre of the figure, and a few 
more deeply placed nuclei maybe found. This is comparable 
to that part which, in the chick, was termed by Balfour “ the 
germinal wall” [g.w.]. Beneath the plug of epiblast {c. p.) 
is a deeply staining and very finely granular matei’ial (y.), 
which is the just-covered superficial layer of the lower pole 
of the yolk. Even as it is, the resemblance between this 
structure and the so-called “ archenteric knot of Oruitho- 
rhynchus” is sufficiently marked to cause one to view with 
suspicion the interpretation placed upon that spot by Wilson 
and Hill. But if we imagine a slightly more advanced con- 
dition, if we imagine the coalescence of the germinal wall, 
either by a gradual closing of the ring or by a differentiation 
of the smalt remaining piece of pui’e yolk into germinal wall, 
the resemblance between the two structures would be even 
more marked. 
In fig. 3 I have drawn a diagram from this figure and 
adopted the same method of indicating the layers as Wilson 
and Hill use in their text-fig, 4, p. 51, using the same 
lettering but attaching to them my interpretation. Thus ect. 
is the epiblast or ectoderm in each case, ent. the hypoblast. 
The epiblast {ect.) is obviously in continuation with the 
cell-plug (c.p.), which is seen in Tropidonotus to be the 
thickened coalesced margin of the epiblast of the blastoderm 
