ARCHENTERIC KNOT

Tropidonotus and the "Archenteric Knot" of Ornithorhynchus.

By

Richard Assheton, M.A.

With Plate 33.

In my paper in a recent number of this Journal, 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 Royal Society,' vol. cxcix, and the interpretation placed by them thereon. A remarkable spot found by these authors and named by them "primitive or archenteric knot" exists at an early stage of the blastocyst of Ornithorhynchus, a stage before the upraising of the neural folds, at some distance in front of the 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.

I ventured to suggest that this spot had nothing to do with archenteron formation, and that it might be a quite erroneous conclusion to identify it with the anterior end of the primitive streak of later times. I argued at some length to show from their description that their interpretation is not tenable. As an alternative I offered the suggestion that the spot in question is the morphological lower pole of the egg, and gave figures illustrating a comparison between Ornithorhynchus as described by Wilson and Hill and a Sauropsidan, such as the sparrow, postulating a complete growth round by the edge of the blastoderm at an earlier stage in the Prototherian than in the Sauropsidan egg in correlation with a smaller quantity of yolk. The fact that there is a very well-marked eccentricity— I mean that this spot is not diametrically opposite to the centre of the "embryo" in Ornithorhynchus as it is in the sparrow, but as it is not in the rabbit-did not seem to me to be a serious objection.

Since making this criticism I have come across a series of sections which I cut through the lower pole of the egg of Tropidonotus natrix some years ago, but which I had forgotten at a stage represented by the outline drawing (Pl. 33, fig. 1).

I find I have also in toto the same spot indicating the coalesced edges of the blastoderm of another egg of the same snake, and the appearances of these specimens are such as to add very considerably to the degree of probability that the suggestion I made will turn out to have been well founded.

The resemblance between the drawings of sections taken through the centre of this area (fig. 2) and Wilson and Hill's text-fig. 4 (p. 51) of their section through the archenteric knot of Ornithorhynchus is most striking. It must be remembered that at this stage in the snake's egg the "blastocyst cavity" is still nearly filled with yolk, which is not shown in my drawing, as the loose yolk has been all washed off during the preparation of the specimen, this loose yolk corresponding to the "tolerably large remainder of the original yolk as a more or less coherent mass, lying free within

ARCHENTERIC KNOT'

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 Tropidonotus, 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.). Beneath this there is, towards the periphery, the hypoblast (hy.), consisting of a reticulum containing many yolk-spherules (y. g.), 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 may be 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 material (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 Ornithorhynchus" 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 condition, if we imagine the coalescence of the germinal wall, either by a gradual closing of the ring or by a differentiation of the small remaining piece of pure 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