(249) F. M. Balfour. _A Monograph on the development of Elasmobranch Fishes_. London, 1878. (250) A. Dohrn. _Der Ursprung d. Wirbelthiere und d. Princip. d. Functionswechsel._ Leipzig, 1875. (251) E. Haeckel. _Schöpfungsgeschichte._ Leipzig. _Vide_ also Translation. _The History of Creation._ King and Co., London, 1876. (252) E. Haeckel. _Anthropogenie._ Leipzig. _Vide_ also Translation. _Anthropogeny._ Kegan Paul and Co., London, 1878. (253) A. Kowalevsky. "Entwicklungsgeschichte d. Amphioxus lanceolatus." _Mém. Acad. d. Scien. St Pétersbourg_, Ser. VII. Tom. XI. 1867, and _Archiv f. mikr. Anat._, Vol. XIII. 1877. (254) A. Kowalevsky. "Weitere Stud. üb. d. Entwick. d. einfachen Ascidien." _Archiv f. mikr. Anat._, Vol. VII. 1871. (255) C. Semper. "Die Stammesverwandschaft d. Wirbelthiere u. Wirbellosen." _Arbeit. a. d. zool.-zoot. Instit. Würzburg_, Vol. II. 1875. (256) C. Semper. "Die Verwandschaftbeziehungen d. gegliederten Thiere." _Arbeit. a. d. zool.-zoot. Instit. Würzburg_, Vol. III. 1876-1877. CHAPTER XIII. GENERAL CONCLUSIONS. I. THE MODE OF ORIGIN AND HOMOLOGIES OF THE GERMINAL LAYERS. It has already been shewn in the earlier chapters of the work that during the first phases of development the history of all the Metazoa is the same. They all originate from the coalescence of two cells, the ovum and spermatozoon. The coalesced product of these cells--the fertilized ovum--then undergoes a process known as the segmentation, in the course of which it becomes divided in typical cases into a number of uniform cells. An attempt was made from the point of view of evolution to explain these processes. The ovum and spermatozoon were regarded as representing phylogenetically two physiologically differentiated forms of a Protozoon; their coalescence was equivalent to conjugation: the subsequent segmentation of the fertilized ovum was the multiplication by division of the organism resulting from the conjugation; the resulting organisms, remaining, however, united to form a fresh organism in a higher state of aggregation. In the systematic section of this work the embryological history of the Metazoa has been treated. The present chapter contains a review of the cardinal features of the various histories, together with an attempt to determine how far there are any points common to the whole of these histories; and the phylogenetic interpretation to be given to such points.