THE “ AECHEINTEEIC KNOT ” OF OENITHOEHYNCHUS. 635 
or opposes tlie view that the Prototherian egg is derived 
from an egg of the Sauropsidan type may be open to question. 
There can be very little doubt that, if such has been its 
origin, there would be a condition as suggested. But it 
would be hazardous to assert that those are the only circum- 
stances under which such a condition could have arisen. 
One can only say that the condition in all essential features 
is identical with that of the Saui’opsidan type, and is totally 
unlike that of Amphibia, Teleosteans, Dipnoi, Elasmobranchs, 
or Cyclostomes. To my mind it appears to form strong 
evidence in confirmation of the validity of the association of 
the Reptilia, Aves, and Mammalia in one distinct group — 
the Amniota. 
Fig. 5 shows the cellular character of the cell-plug, the 
thin general layer of epiblast continuous with the thickened 
margin, and the hypoblast and yolk-spherules, many dis- 
integrated, foi’ming the germinal wall or marginal zone of 
Wilson and Hill. 
Aros, 
Isle of Mull, 
August, 1909. 
Papers referred to in the Test. 
1. Assheton, R. — “ Professor Hubreclit's Paper on the Ontogenetic 
Phases of Mammalian Development ; An Appreciation and 
Respectful Criticism,” ‘ Quart. Journ. Micr. Sci.,’ vol. 51, 1909. 
2. Wilson, J. T., and Hill, J. P.— “ Primitive Knot and Early Gastru- 
lation Cavity Coexisting with Independent Primitive Streak in 
Oniithorhynchus,” ‘ Proc. Roy. Soc. London,’ vol. Ixxi, 1903. 
— “ Observations on the Development of Ornithorhyn- 
chus,” ‘Phil. Trans. Roy. Soc. London,’ ser. B, vol. cxcix, 1907. 
8. 
