Art. XX.—On the Microscopical Structure of the Egg-shell of the Moa.
[Read before the Wellington Philosophical Society, 26th August, 1871.]
Having been kindly supplied by Dr. Hector with some broken fragments of the Moa's egg, I have now the honour to lay before the Society the results of a microscopical examination which I have made of them; I must, however, first observe that I have been informed that the structure of the Moa's egg was described many years ago, but, as I cannot find any trace of that description, I hope I shall escape the accusation of making you listen to a repetition of well known facts.
About four years ago Dr. Blasius published an extensive and detailed account of the structure of the shell of birds' eggs. This account I have not seen, but I am aware that he arrived at the conclusion that the microscopic differences are neither constant nor reliable, and cannot be used for purposes of classification. There is, however, one constant feature that distinguishes the eggs of the Struthious birds from the rest of the class, or the Carinate birds. This is, that in the eggs of the Struthious birds the carbonate of lime shows a prismatic structure, while in the eggs of the Carinate birds the prismatic structure is absent.
I believe that I have examined the egg-shell of more than one species of Moa, but I find the structure to be fundamentally the same in all, and that the differences in different portions of the same specimen are quite as great as in any two different specimens. The shell, when not abraded, is of a pale yellow colour, smooth, and irregularly pitted on the outside with dots and linear markings, sometimes 0.08 of an inch in length. (Pl. IX., Fig. 1). On some fragments the markings were all straight, in others they were nearly all curved, while in others again both straight and curved markings occurred together. Round dots were on all the specimens. In appearance they are more like the egg of the South American Rhea than any other that I know. The specific gravity I found to be 2.714. On dissolving in dilute hydrochloric acid no residue was left. The shell is about 0.07 of an inch in thickness, and is divided into two layers, each presenting a different structure (Fig. 2).
The outer layer (Fig. 2 a) forms about two thirds of the whole thickness, and is composed of a large number of thin laminæ, arranged parallel to the surface of the egg. Each of these laminæ appears to be void of structure when viewed under a power of 400 diameters, being made up of numerous points variously aggregated together into clouds (Fig. 3), very similar in appearance to mucilage.
The inside layer is totally different, showing an irregular columnar structure, which very easily breaks up (Fig. 2 b). Sections made parallel to the surface of the egg, or at right angles to the columns, show that each column contains many, more or less complete, triangular prisms of carbonate of lime. Under a low power these prisms appear to have a radiating arrangement from a nucleus, and often there are two, three, or four nuclei in each column (Fig. 4), but under a higher power this disappears. This appearance is owing to the prisms being collected more thickly in the centre of the column, and to many of the imperfectly formed ones having a well formed apex pointing inwards, while the base of the triangle is undefined, and shades off outwards into a brush of very fine spiculæ (Fig. 5).
The prisms appear to be always triangular, and to vary in section from an equilateral to an isosceles triangle, in which the base is about half the length of one of the sides. These, however, might all be produced by variously inclined sections of an equilateral triangular prism. The length of the sides vary from very small up to 0.003 of an inch, which is the longest that I have measured.
The egg of the Kiwi (Apteryx) shows none of this prismatic structure, but is in every way similar to that of the common fowl, and we have, therefore, here further evidence that the Moa belongs to the Struthious type, where it has always been placed, while the Kiwi, in the structure of its egg-shell belongs to the Carinate type of birds.
Note, Aug. 29th, 1871.—Since reading this paper I have found the following notice in the “Zoological Record” for 1869, part I., page 103:— “Dinornis.—The structure of its egg-shell is essentially similar to that of other Struthiones, and agrees most nearly with Rhea.—W. von Nathusius, Zeitschr. wissensch. Zool. XX., p. 118.” Also on p. 104, “Apteryx, in the structure of its egg-shell, does not much agree with other Struthiones.”