On certain aspects of human biology / by C.J. Bond.

  • Bond, C. J. (Charles John), 1856-1939
Date:
1932
Catalogue details

Licence: In copyright

Credit: On certain aspects of human biology / by C.J. Bond. Source: Wellcome Collection.

    11/192 (page 7)
    rather than a preformative theory of embryonic develop- ment. If we start then with the assumption that the development of a recently introduced character, such for instance as the extra toe, (when a four-toed breed of fowls is crossed with a five-toed breed), is controlled by the same heredity process as older established racial characters, e.g. colour pattern are controlled, during embryonic develop- ment,—then it is probable that an enquiry into- these and other examples of hemilateral asymmetry, such as gynan- dromorphism in birds and insects, may throw additional light on the actual process and method of embryological growth. For whatever be the nature of the process, (whether it be preformative or epigenetic) which starts the develop- ment of the fertilised egg cell on a bilateral plan of growth, the fact that an old established racial character is, for the most part, distributed equally to the two halves of the organism, while a newly introduced character, (such as an extra toe), is often distributed unequally, suggests the idea that the new factor, or gene, fuses with difficulty with the older established complex. In other words, that the old and new factors tend to segregate during the process of cell division which underlies the bilateral development of the embryo. The result being that asymmetrical dis- tribution of the extra toe character occurs on the two sides of the body. It looks indeed as if a condition of unstable equilibrum has been set up in the genetic machinery which controls embryological development, by the introduction of a foreign element or character into an old established racial genetic complex, and that this disturbance of equilibrum is in some way associated with the abnormal distribution of the character in question, to the two sides of the body. It fact it is not altogether fanciful to regard the individual vertebrate organism as built up of two half individuals, mirror images of each other, fused along the longitudinal median plane, just as we regard a serially segmented organism, like a worm, as built up of a number