EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 253 
streak, and called by Wilson and Hill the “primitive or 
archenteric knot,” as tlie equivalent of the protochordal 
Avedge of his nomenclature. Nevertheless I think Hubrecht 
is not very happy about it, as he says: “However, the data 
concerning the earliest appearance of this protochordal plate 
in Oruithorhynchus are too scanty than that I have ventured 
to mention it when in the preceding pages we discussed the 
protochordal plate. And it seems advisable on this point to 
await yet further researches on these rare mammals, of which 
it is so very difficult to obtain the required developmental 
stages.” 
In spite of this no doubt admirable caution, I venture to 
suggest that the object called primitive knot by Wilson 
and Hill (pr. K., text-fig. 7, Wilson and Hill) has nothing at 
all to do with either gastrulatiou or primitive streak, but is 
really the morphological vegetative pole of the egg. 
I have tried to make my meaning clear by the accompany- 
ing diagram (text-fig. 5). 
A. represents a meroblastic egg, such as that of a sparrow. 
In this the “ liquefaction ” of the yolk is seen very obviously 
to result in an accumulation of fluid between the blastoderm 
and the yolk mass, which forms a mass at the lower pole of 
the whole egg. The diagram is, however, not an exact 
representation. The epiblast, ep., is shown to have nearly 
surrounded the yolk, a condition which is in reality not 
attained until a much later stage of development of the 
embryo itself than is indicated by the upper pole of the egg 
in my diagram, which represents a sagittal section of an 
early stage, perhaps eighteen hours or so of incubation. 
Let us imagine, however, this growth of the epiblast, ep., 
to take place very early (so that the yolk is completely 
enveloped by it) and the thickened edges to meet and fuse 
early. Let us imagine that the yolk is much reduced, and its 
place taken by fluid, so that the nucleated margins {gw.), the 
so-called germinal walls, have met and fused also ; we should 
then have a condition not unlike the fig. B, except that in 
fig. B the part representing the coalesced blastoderm 
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