THE “AEGHENTEEIC KNOT ” OF OENITHOEHYNCHUS. 631 
Tropidonotus and the “Archenteric Knot ” of 
Ornithorhynchus. 
By 
Kicliai'd Asslietoii, M.A. 
With Plate 33. 
In my paper in a recent number of this Jouimal, in which 
I discussed Professor Hubrecht’s memoir upon the ontogenetic 
phases of mammalia, I referred to the peculiar condition of 
the egg of Ornithorhynchus as described by Wilson and Hill 
in the ' Philosophical Transactions of the Eoyal Society,’ 
vol. cxcix, and tlie interpretation placed by them thereon, 
A remarkable spot found by these authors and named by 
them “primitive or archenteric knot” exists at an eaidy stage 
of the blastocyst of Ornithorhynchus, a stage before the 
upraising of the neural folds, at some distance in front of 
tlie primitive streak which is present in a form perfectly 
typical of mammalia. 
The whole blastocyst at this period is “occupied by mainly 
fluid contents,” the more solid yolk of the previous stage 
(6 mm. in diameter) having become partly disintegrated by 
absorption of fluid from the uterus by this time, when the 
diameter of the blastocyst has attained 10 mm, 
Wilson and Hill regarded this spot as representing an early 
stage in the development of the archenteron, and attempted 
to identify it with the “Hensen knot” of the later period. 
Considerable gaps exist in the material, so that they were not 
able to trace accurately either the origin or the fate of this 
structure. 
