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. Semper--Segmentalorgane bei ausgewachsenen Haien. _Centralblatt f. Med. Wiss._ 1874, No. 52. Semper--Das Urogenitalsystem der höheren Wirbelthiere. _Centralblatt f. Med. Wiss._ 1874, No. 59. Semper--Stammesverwandtschaft d. Wirbelthiere u. Wirbellosen. _Arbeiten aus Zool. Zootom. Inst._ Würzburg. II Band. Semper--Bildung u. Wachstum der Keimdrüsen bei den Plagiostomen. _Centralblatt f. Med. Wiss._ 1875, No. 12. Semper--Entw. d. Wolf. u. Müll. Gang. _Centralblatt f. Med. Wiss._ 1875, No. 29. Alex. Schultz--Phylogenie d. Wirbelthiere. _Centralblatt f. Med. Wiss._ 1874, No. 51. Spengel--Wimpertrichtern i. d. Amphibienniere. _Centralblatt f. Med. Wiss._ 1875, No. 23. Meyer--Anat. des Urogenitalsystems der Selachier u. Amphibien. _Sitzb. Naturfor. Gesellschaft._ Leipzig, 30 April, 1875. F. M. Balfour--Preliminary Account of development of Elasmobranch fishes. _Quart. Journ. of Micro. Science_, Oct. 1874. (This edition, Paper V. p. 60 _et seq._) W. Müller--Persistenz der Urniere bei Myxine glutinosa. _Jenaische Zeitschrift_, 1873. W. Müller--Urogenitalsystem d. Amphioxus u. d. Cyclostomen. _Jenaische Zeitschrift_, 1875. Alex. Götte--_Entwicklungsgeschichte der Unke (Bombinator igneus)._ The following paper is an attempt to give a consecutive history of the origin of this system of organs in vertebrates and of the changes which it has undergone in the different orders.