Footnote 115: "Die Embryonal Anlage der Selachier. Vorläufige Mittheilung," _Centralblatt f. Med. Wiss._ No. 33, 1875. EXPLANATION OF PLATE 6. Fig. 1. Section through the germinal disc of a ripe ovarian ovum of the Skate. _gv._ germinal vesicle. Fig. 2. Surface-view of a germinal disc with two furrows. Figs. 3, 4, 5. Surface-views of three germinal discs in different stages of segmentation. Fig. 6. Section through the germinal disc represented in fig 3. _n._ nucleus; _x._ edge of germinal disc. The engraver has not accurately copied my original drawings in respect to the structure of the segmentation furrows. Figs. 6_a_ and 6_b_. Two furrows of the same germinal disc more highly magnified. Fig. 6_c_. A nucleus from the same germinal disc highly magnified. Fig. 7. Section through a germinal disc of the same age as that represented in fig. 4. _n._ nucleus; _nx._ modified nucleus; _nx´._ modified nucleus of the yolk; _f._ furrow appearing in the yolk around the germinal disc. Figs. 7_a_, 7_b_, 7_c_. Three segments with modified nuclei from the same germinal disc. Fig. 8. Section through a somewhat older germinal disc. _ep._ epiblast; _n´._ nuclei of yolk. Figs. 8_a_, 8_b_, 8_c_. Modified nuclei from the yolk from the same germinal disc. Fig. 8_d_. Segment in the act of division from the same germinal disc. Fig. 9. Section through a germinal disc in which the segmentation is completed. It shews the larger collection of cells at the embryonic end of the germinal disc than at the non-embryonic. _ep._ epiblast. CHAPTER III. FORMATION OF THE LAYERS. In the last chapter the blastoderm was left as a solid lens-shaped mass of cells, thicker at one end than at the other, its uppermost row of cells forming a distinct layer. There very soon appears in it a cavity, the well-known segmentation cavity, or cavity of von Baer, which arises as a small space in the midst of the blastoderm, near its non-embryonic end (Pl. 7, fig. 1.). This condition of the segmentation cavity, though already[116] described, has nevertheless been met with in one case only. The circumstance of my having so rarely met with this condition is the more striking because I have cut sections of a considerable number of blastoderms in the hope of encountering specimens similar to the one figured, and it can only be explained on one of the two following hypotheses. Either the stage is very transitory, and has therefore escaped my notice except in the one instance; or else the cavity present in this instance is not the true segmentation cavity, but merely some abnormal structure. That this latter explanation is a possible one, appears from the fact that such cavities do at times occur in other parts of the blastoderm. Dr Schultz[117] does not mention having found any stage of this kind. Footnote 116: _Qy. Journal of Microsc. Science_, Oct. 1874. [This Edition, No. V.]