December, 1955 ] H. EPSTEIN 315
comes into operation ; thus, if the embryo is slow in developing, the abnormality
is marked, whereas if the factor takes effect only at an advanced stage of develop-
ment, the abnormality is slight.
In Homo sapiens the neural spines of the 2nd to 5th cervical vertebrae are com-
monly bifid, while bifurcation in the 6th cervical vertebra is rarer, and in the 7th
entirely absent. This, however, applies only to Europeans; in the Australian
aborigines, Negroes, Andamans, Tasmans, Hottentots, Bushmen, the natives of
Tierra del Fuego and the Senoi only the epistropheus is bifid, while the neural spines
of the 3rd to 5th cervical vertebrae are but seldom terminally cleft. Also in
Japanese, bifidity of the cervical neural spines is less common than in Europeans,
and the spines are shorter.
In the chimpanzee the epistropheus is nearly always bifid ; in the gibbon this
character is frequent, but in the orung-utan, the gorilla and the more primitive
primates it is of rare occurrence. In all apes and monkeys the processus spinosi
of the other cervical vertebrae are normally undivided, although exceptions from
this rule have been described [Von Eggeling, 1922a].
Mijsberg [1926] points out that phylogenetically spina bifida is a specialised
character in relation to the more generalised undivided processus spinosi, while
ontogenetically the neural spines evolve from two separate bases. In the most
primitive type (Plate VIII, Fig. 6-1), the neural spine develops from the cartilaginous
tissue formed by the terminal union of the bilateral neural arches which are devoid
of any protuberances at the tips. In the more specialised types (Plate VIII Fig.
6-2a and 2b), the tips of the bilateral neural arches are thickened, and these pro-
tuberances share in the basis of the neural spine, the major part of which evolves,
however, from the cartilage formed by the coalescence of the neural arches. In the
third type (Plate VIII, Fig. 6-3), the protuberances at the tips of the neural arches
are so strongly developed that the median tip formed by the fusion of the neural
arches rises above and absorbs the lateral hyperapophyses only at a comparatively
late stage of development. In the fourth type (Plate VIII, Fig. 6-4), the protuber-
ances grow into long processes which subsequently unite into the single spine, with
the median cartilage retaining but slight importance in the formation of the
process. In the fifth type (Plate VIII, Fig. 6-5), the lateral protuberances fuse at
first at the base only, the single spine evolving after a bifid stage. In the sixth
type (Plate VIII, Fig. 6-6), the fusion of the two original halves of the processus
spinosus remains restricted to the base throughout the life of the individual, with
spina bifida as the result.
Von Eggeling [1922b] suggests that in analogy with many other skeletal crests
and ridges the development of bifid spinous processes may be due to the influence
of the muscles and sinews attached to the spinous processes ; while in the case of
the dorsal vertebrae of zebu cattle, Duerst [1931] regards the bifurcation of the
spinous processes as functional of the weight of the hump. But in view of the fact
that certain humped zebus are devoid of the bifid character, whereas spina bifida
may occur in unhumped sanga specimens, the validity of this theory is doubtful.
Mijsberg [1926] ascribes the bifurcation of the neural spines of the cervical vertebrae
in Homo to a factor inhibiting the full development of the median cartilaginous
tissue formed by the fusion of the bilateral neural arches, with the result that the