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.
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(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.
(246) C. Kupffer. “Die Entstehung d. Allantois u. d. Gastrula d. Wirbelthiere.” Zool. Anzeiger, Vol. II. 1879, pp. 520, 593, 612.
(247) R. Remak. Untersuchungen üb. d. Entwicklung d. Wirbelthiere, 1850-1858.
(248) A. Rauber. Primitivstreifen u. Neurula d. Wirbelthiere. Leipzig, 1877.
[99] A parallel to the unpaired medullary plate of most Chordata is supplied by the embryologically unpaired ventral cord of most Gephyrea and some crustacea. In these forms there can be little doubt that the ventral cord has arisen from the fusion of two originally independent strands, so that it is not an extremely improbable hypothesis to suppose that the same may have been the case in the Chordata.
CHAPTER XII.
OBSERVATIONS ON THE ANCESTRAL FORM OF THE CHORDATA.
The present section of this work would not be complete without some attempt to reconstruct, from the materials recorded in the previous chapters, and from those supplied by comparative anatomy, the characters of the ancestors of the Chordata; and to trace as far as possible from what invertebrate stock this ancestor was derived.