_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 stages.
VII. ON THE ORIGIN AND HISTORY OF THE URINOGENITAL ORGANS OF VERTEBRATES[26].
Footnote 26: From the _Journal of Anatomy and Physiology_, Vol. X. 1875.
Recent discoveries[27] as to the mode of development and anatomy of the urinogenital system of Selachians, Amphibians, and Cyclostome fishes, have greatly increased our knowledge of this system of organs, and have rendered more possible a comparison of the types on which it is formed in the various orders of vertebrates.
Footnote 27: The more important of these are:--
Semper--Ueber die Stammverwandtschaft der Wirbelthiere u. Anneliden. _Centralblatt f. Med. Wiss._ 1874, No. 35.