_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.