On certain problems of vertebrate embryology / by John Beard.

  • John Beard
Date:
1896
Catalogue details

Licence: Public Domain Mark

Credit: On certain problems of vertebrate embryology / by John Beard. Source: Wellcome Collection.

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

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    That the organs may to a greater or less degree in- dividually recapitulate their phylogeny is something that I, for one, have never denied. But this confirms the truth, not of the recapitulation theory, but of the maxim ''omne organum ex organo'. If this be Keibel's meaning when speaking of a Monotreme stage, i. e. if he Hmit it to the existence of a cloaca along with a certain condition of rectum and urinogenital organs — and this appears to be so — no objection can, at present at any rate, be offered to the validity of this view. Any supposed recapitulation of various stages per- manent in lower forms in the ontogeny of, say, a higher Mammal can easily be disposed of by a simple reductio ad absurdum. For the sake of argument let such a series of hypo- thetical stages as those suggested by Haecke 1 be taken; although, as must be admitted, it would be unfair to Vertebrate embryologists to assume that any one of them had announced his adhesion to this scheme. But, unfortunately, there is no other series that can be got hold of, for no one else, except Oppel^), has committed himself to anything at all definite. The actual existence of any such series in the development of a higher Vertebrate is disproved by the circumstance that, when the embryo may be supposed to be in any one of l) While rejecting the applicability of the recapitulation theory to the Vertebrata, Op pel, as cited by Keibel, still tliinks the existence of a Vorfischstadiums, eines Fischstadiums, eines Land- thier- und eines Proamniotenstadiums possible during the develop- ment of a Mammal! (O p p el, A., Vergleicliung des Entwickelungs- grades der Organe. Jena 1891). Comment is quite superfluous.
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