Mendelism and evolution / by E.B. Ford.

  • Ford, E. B. (Edmund Briscoe), 1901-1988.
Date:
1931
Catalogue details

Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Mendelism and evolution / by E.B. Ford. Source: Wellcome Collection.

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    66 MENDELISM AND EVOLUTION in the presence of the maternal nucleus and gene-com¬ plex, and this may be responsible for directing the early development, perhaps before the new diploid nucleus can exercise its effect. The strong probability of this is indicated by the work of Tennent in fertilizing the normal (nucleated) eggs of Cidaris with the sperm of Lytechinus (Morgan, 1927, p. 635). In the latter genus development proceeds much more rapidly than in the former, and the origin of the mesenchyme cells is different. In the cross- fertilized egg no effect of the sperm is observable before gastrulation begins, for the rate of cleavage is not hastened and the blastula has the appearance tjrpical of Cidaris. The origin of the mesenchyme, however, is intermediate between the two parental types. In the hybrids it is not formed until after gastrulation has begun, and by that time the cytoplasmic-nuclear rela¬ tion has been restored to normal. Godlewski (1925) has shown that in the Echinoderms this is usually about 7:1. In the mature egg it is 400 : 1, in the four-cell stage of the embryo 18: 1, in the sixty-one-cell stage 12 : 1, while in the fulty-formed blastula it has returned to its normal value. It may well be, therefore, that the nucleus is unable to control ^ volume of cytoplasm relatively so great as that found in the earliest stages of development, but that it asserts its effect when the normal conditions are restored. The course of development up to that time may be determined by the maternal gene-complex. We have no clear evidence that this is so, but such an assumption is quite a legitimate one in view of the fact that instances are actually known in which the parental gene-complex imposes an effect upon the next genera¬ tion as, for example, the important researches of Diver (1925) on Limnœa have clearly shown.
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