In the Bird the representative of the horizontal furrow appears relatively much earlier. It is formed when there are eight segments marked out on the surface of the germinal disc[100]. From Oellacher's[101] account of the segmentation in the fowl[102] it seems certain, as might be anticipated, that this furrow is nearly parallel to the surface of the disc, so that it cuts the earlier formed vertical furrows and causes the segments of the germinal disc to be completely circumscribed below as well as at the surface. In the Elasmobranch egg this is not the case; so that, even after the smaller central segments have become separated from the outer ring of larger ones, none of the segments of the disc are completely circumscribed, and only appear to be so in surface views (vide Pl. 6, fig. 6). Segmentation in the Elasmobranch egg differs in the following particulars from that in the Bird's egg:
(1) The equivalent of the horizontal furrow of the Batrachian egg appears much later than in the Bird.
(2) When it has appeared it travels inwards much more slowly.
Footnote 100: Vide _Elements of Embryology_, p. 23.
Footnote 101: _Stricker's Studien_, 1869, Pt. I, Pl. II. fig. 4.
Footnote 102: Unfortunately Professor Oellacher gives no account of the surface appearance of the germinal discs of which he describes the sections. It is therefore uncertain to what period his sections belong.
As a result of these differences, the segments of the germinal disc of the Birds' eggs are much earlier circumscribed on all sides than those of the Elasmobranch egg.
As might be expected, the segmentation of the Elasmobranch egg resembles in many points that of Osseous Fishes (vide Oellacher[103] and Klein[104]). It may be noticed, that with Osseous as with Elasmobranch Fishes, the furrow corresponding with the horizontal furrow of the Amphibian's egg does not appear at as early a period as is normal. The third furrow of an Osseous Fish egg is parallel to one of the first formed pair.
Footnote 103: _Zeitschrift für Wiss. Zool._ Bd. XXII. 1872.
Footnote 104: _Monthly Microscopical Journal_, March, 1872.
In Oellacher's[105] figures, Pl. 23, figs. 19-21, peculiar beadings of the sides of the earlier formed furrows are distinctly shewn. No mention of these is made in the text, but they are unquestionably similar to those I have described in the Elasmobranch furrows. In the case of Elasmobranchii I pointed out that not only were the sides of the furrow beaded, but that there appeared in the protoplasm, close to the furrows, peculiar vacuole-like cavities, precisely similar to the cavities which were the cause of the beadings of the furrows.