EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 255 
margins (epiblast and germinal walls) are eccentric. Now, 
is this a possible explanation of tlie primitive knot of Orni- 
tliorhynclius ? 
Fig. B represents a sagittal section of the monotreme egg, 
drawn from Wilson and Hill’s description, except that the 
layers are shown too thick, and the primitive knot, M., is 
larger than it should be. 
On this interpretation the section illustrated diagrammatic- 
ally by Whlson and Hill, p. 51 of their paper, should be lettered 
thus : “ c.f. cell plug ” = fused edges of epiblast ; “ ent. yolk 
entoderm” = hypoblast; “ vi.z. marginal or cortical zone” 
= germinal wall or only remains of general yolk mass not 
yet liquefied ; “ c.z. central or more cellular zone ” is probably 
also fused edges of epiblast, the edge more or less inter- 
mingled with the germinal wall yolk- cel Is. 
The reason why it is not diametrically opposite the centre 
of the embryonal area is to be sought for chiefly in the greater 
expansion of the posterior arc or segment of the sphere. 
Such a condition is to be found in the rabbit’s blastocyst, but 
in a very much less marked degree. If a blastocyst of a 
rabbit is taken out of the uterus at about seven days sixteen 
hours, i.e. just before it becomes attached by its lower pole 
to the walls of the uterus, it will be found to be of an oval 
form, with a smaller and acuter anterior and a larger and 
obtuser posterior end, and an area more or less ventral which 
will be devoid of hypoblast. The centre of this area is not 
diametrically opposite the centre of the developing embryonal 
area, but more towards the anterior end (vide Assheton, 1894, 
PI. 17, fig. 47). 
My suggestion is that the position of the primitive knot is 
due to a still more marked effect of a similar cause, the cause 
being the activity of the primitive streak or deuterogenetic 
centre, which allows of the more rapid expansion of the 
posterior segment of the blastocyst. 
On this interpretation the primitive knot is of little import- 
ance, and the Ornithorhynchus egg appeal’s as a condition 
intermediate between that of a reptilian and that of an avine 
