A Study of Mutations in Evolution - IV. Ontogeny of the Equine Foot

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1. Three toes are present on each foot of the embryo of the modern horse. Ontogeny here seems to reflect phylogeny. 2. The dimensions of the lateral toes of the embryo are quantitatively distinguishable from those of the primitive foot as early as the crown-rump 20 mm. stage, being (by extrapolation) then half as large (see Fig. 1). 3. Subsequent to this stage the exponential growth-rate of the lateral digits (with reference to the central digit) is substantially identical in the tridactyl and monodactyl foot. 4. In these modern cases of polydactyly which are described as atavistic we find the lateral digits to have reverted to ancestral dimensions. The genetic basis of this reversal is not clear. 5. In the ontogeny of the equine foot there is no evidence of recapitulation, since at the earliest known stage the analgen present a size difference. Moreover, this difference is algebraically constant, although qualitatively less conspicuous in the younger individuals than in the older. 6. Ontogeny may repeat or overstep phylogeny in some one structure or region (as in the dimensions of the skull), yet diverge completely in some other organ (such as the foot) of the same organism. 7. It is inferred that certain basic genes present in the ancestor may continue to act unchanged in the modern embryo, whereas other genes which have undergone mutation will subsequently affect embryogenesis in a novel fashion. For this reason the relationship between ontogeny and phylogeny will require as many definitions as there are organs or genes involved.