The advantage of secondary respiratory organs during foetal life, in addition to the yolk-sack, is evinced by the fact that such organs are very widely developed in the Ichthyopsida. Thus in Elasmobranchii we have the external gills (cf. p. 62). Amongst Amphibia we have the tail modified to be a respiratory organ in Pipa Americana; and in Notodelphis, Alytes and Cæcilia compressicanda the external gills are modified and enlarged for respiratory purposes within the egg (cf. pp. 140 and 143). The Amnion. The origin of the amnion is more difficult to explain than that of the allantois; and it does not seem possible to derive it from any pre-existing organ. It appears to me, however, very probable that it was evolved _pari passu_ with the allantois, as a simple fold of the somatopleure round the embryo, into which the allantois extended itself as it increased in size and became a respiratory organ. It would be obviously advantageous for such a fold, having once started, to become larger and larger in order to give more and more room for the allantois to spread into. The continued increase of this fold would lead to its edges meeting on the dorsal side of the embryo, and it is easy to conceive that they might then coalesce. To afford room for the allantois close to the surface of the egg, where respiration could most advantageously be carried on, it would be convenient that the two laminæ of the amnion--the true and false amnion--should then separate and leave a free space above the embryo, and thus it may have come about that a separation finally takes place between the true and false amnion. This explanation of the origin of the amnion, though of course hypothetical, has the advantage of suiting itself in most points to the actual ontogeny of the organ. The main difficulty is the early development of the head-fold of the amnion, since, from the position of the allantois, it might have been anticipated that the tail-fold would be the first formed and most important fold of the amnion. BIBLIOGRAPHY. (239) F. M. Balfour. "A comparison of the early stages in the development of Vertebrates." _Quart. J. of Micr. Science_, Vol. XV. 1875. (240) F. M. Balfour. "A monograph on the development of Elasmobranch Fishes." London, 1878. (241) F. M. Balfour. "On the early development of the Lacertilia together with some observations, etc." _Quart. J. of Micr. Science_, Vol. XIX. 1879. (242) A. Götte. _Die Entwicklungsgeschichte d. Unke._ Leipzig, 1875. (243) W. His. "Ueb. d. Bildung d. Haifischembryonen." _Zeit. f. Anat. u. Entwick._, Vol. II. 1877. Cf. also His' papers on Teleostei, Nos. 65 and 66. (244) A. Kowalevsky. "Entwick. d. Amphioxus lanceolatus." _Mém. Acad. des Sciences St Pétersbourg_, Ser. VII. Tom. XI. 1867. (245) A. Kowalevsky. "Weitere Studien üb. d. Entwick. d. Amphioxus lanceolatus." _Archiv f. mikr. Anat._, Vol. XIII. 1877.