_al._ Cavity of alimentary canal. _bl._ Blastoderm. _ch._ Notochord. _ep._ Epiblast. _em._ Embryo. _f._ Formative cells. _hy._ Hypoblast. _ll._ Lower layer cells. _m._ Mesoblast. _n._ Nuclei of yolk of Selachian egg. _nc._ Neural canal. _sg._ Segmentation cavity. _x._ Point where epiblast and hypoblast are continuous at the mouth of the alimentary involution. This point is always situated at the tail end of the embryo. _yk._ Yolk. Epiblast is coloured blue, mesoblast red, and hypoblast yellow. The lower layer cells before their separation into hypoblast and mesoblast are also coloured green. A I, A II, A III. Diagrammatic sections of Amphioxus in its early stages (founded upon Kowalevsky's observations). B I, B II, B III. Diagrammatic longitudinal sections of an hypothetical animal, intermediate between Amphioxus and Batrachians, in its early stages. C I, C II, C III. Diagrammatic longitudinal sections of Bombinator igneus in its early stages (founded upon Götte's observations). In C III the neural canal is completed, which was not the case in B III. The epiblast in C III has been diagrammatically represented as a single layer. D I, D II, D III. Diagrammatic longitudinal sections of an animal, intermediate between Batrachians and Selachians, in its early stages. E I, E II, E III. Diagrammatic longitudinal sections of a Selachian in its early stages. E´. Surface view of the yolk of a Selachian's egg to shew the manner in which it is enclosed by the Blastoderm. The yolk is represented yellow and the Blastoderm blue. F I, F II, F III. Diagrammatic longitudinal sections of a Bird in its early EXPLANATION OF PLATE 6. (X. p. 222.) 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.