The elements of embryology. / By M. Foster ... and Francis M. Balfour.

  • Michael Foster
Date:
1874
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Licence: Public Domain Mark

Credit: The elements of embryology. / By M. Foster ... and Francis M. Balfour. Source: Wellcome Collection.

Provider: This material has been provided by The University of Leeds Library. The original may be consulted at The University of Leeds Library.

    109/336 (page 83)
    IV.] THE AJiIXION. ^3 blastic in origin, having been separated from the epiblast at the epoch of the apparent fusion of the epiblast and mesobkst iti the re-ion of the primitive str<;ak or axia-cord of His. This view, prompted as it evidently is by theoretical considerations, must be regarded as untenable, since the primitive streak 1ms nothmg whatever to do with the permanent body of the enibryo w li® [ecently Romiti {Archiv. Microscop. Anat. x. 1874) has described the Wolthan duct as bemg formed by an involution of the epithelium of the pleuro- pmtoneal cavity, in the form of a longitudinal groove whicli is thrust up into the superior portions of the intermediate ceil-mass, and the communication of which with the pleuropentoneal cavity is speedily obliterated. Such a mode of or^m recommends itself to the embryologfst, inasmuch as it is certainly the way m whu^h, as we shall see, the Wolffian duct is formed in Amphibia and Osseous Fishes. For that very reason however it should be received whh cauuon; al the more since the sections drawn by Romiti, and described as Sh[.h?h^1 Tr'^r^'l-^^^T '^'^ ' considerably later thaniha at which the duct first d.stmctly appears. We hope to be able to shew, in the second pai-t of this work, that the mode of development of the Wolffian duct described above, and which we believe to be the real one, i, not so abnormal as It might at first sight be supposed to be. d,uuormai as 22. The amnion, especially the anterior or head-fold advances m growth very rapidly during the second day, and at Its close completely covers the head and neck of the embryo ; so much so that it is necessary to tear or remove it when the head has to be examined in hardened opaque speci- mens, ihe tail and lateral folds of the amnion, though still progressmg, lag considerably behind the head-fold. ^^I'l TJ^^.°^^®^ ^^^^^^ *hen which occur during the second half of the second day are as follows :— 1. The second and third cerebral vesicles make their appearance behind the first. 2. The optic vesicles spring as hollow buds from the ateral, and the vesicles of the cerebral hemispheres from the Iront portions of the first cerebral vesicle. 3. The first rudiment of the ear is formed as an involu- tion of the epiblast on the side of the hind-brain or third cerebral vesicle. visible ^'''^ cranial flexure become mJl.Vr and especially the splanchnopleuie moiety, advances rapidly backwards; the head of the embryo be^ores diSt! ^'^^'^'^ ^^^^ ^'^^ mJi curvature of the heart increases; the first rudi- ments ot the auricles appear. 7. The circulation of the yolk-sac is completed, o. llie amnion grows rapidly. 6—1