lux in addition to an extra set of one to three toes. Asymmetry is com-
mon. The femur, tibjo-tarsus, and tarso-metatarsus are shortened in em-
bryos with typical diplopodia. About 97 per cent of the chicks have
extra digits in the wing, usually consisting of one phalanx only, and in
about 80 per cent the maxilla is shortened.
Genetics. This remarkable abnormality is inherited as a simple, re-
cessive autosomal character. Heterozygotes mated inter se produced
1,531 embryos, of which 22.7 per cent showed the mutation. The fact
that some of these matings yielded fewer diplopod embryos than the 25
per cent expected was attributed to its suppression by modifying genes
or environmental influences. The symbol dp is suggested.
Diplopodia is lethal to about 98 per cent of the homozygotes, most of
which apparently die during the last week of incubation. Out of 423
diplopods, only 9 were hatched. One of these was raised but proved too
abnormal for reproduction.
Brachydactyly, By
This is a variation in the outer toe. If the digits of the foot be
numbered from the inner to the outermost one as D I, II, III, and IV, the
corresponding numbers of phalanges are normally two, three, four, and
five.
Morphology. In normal fowls D IV is about 10 per cent longer than
D II or a little more, but in brachydactylous ones D IV is as short as D II
or shorter. Four degrees of the condition were recognized by Danforth
(1919).
1. All five phalanges are present, but the fourth and sometimes the
third are shortened.
2. The third and fourth are replaced by a single phalanx, like the
fourth, but larger.
3. The third and fourth are fused; all are reduced. The claw is only
a small fiat scale and the fifth phalanx a nodule into which is inserted the
terminal slip of the flexor profundis tendon.
4. There are only two bones; no nail.
Brachydactyly is recognizable in some embryos at 9 days, in nearly all
by 10, but some cases may be overlooked even in later embryos.
Genetics. The extensive data of Warren (1940) show that brachy-
dactyly is caused by an incompletely dominant autosomal gene, By.
Most of the birds homozygous for this lack the nail and some of the
phalanges of D IV, i.e., they are Danforths grade 4. Heterozygotes are
affected somewhat less severely but are recognizable by shortening of the
toe. In one of Warrens matings 6 per cent of the homozygotes showed
the phenotype characteristic of heterozygotes.