ogised with the
supraoccipital of many mammals, for its upper half arises at first in
isolation as a secondary bone (p. 290).

Reichert objected to the distinction drawn by Koelliker, and denied that
there was either a histological or a morphological difference between
membrane and cartilage bones. It was shown a few years later by H.
Mueller[233] that there was in truth no essential difference in
histological development between the two categories of bone, that the
cartilage cells were replaced by bone cells identical with those taking
part in the formation of membrane bones. The morphological distinction
continued however to be recognised, particularly by the embryologists.
Rathke in his volume of 1861[234] classified the bones of the skull
according to their origin from the primordial cranium or from the
overlying fibrous layer, distinguishing as membrane bones, the
parietals, frontals, nasals, lachrymals, maxillaries and premaxillaries,
jugals, tympanic, parts of the "temporal," vomer, part of the
supraoccipitals in some mammals, and the mandible (with the exception of
the articular in such as have a quadrate bone). Huxley was also inclined
in 1864[235] to recognise the distinction, but he writes with some
reserve:--"Is there a clear line of demarcation between membrane bones
and cartilage bones? Are certain bones always developed primarily from
cartilage, while certain others as constantly originate in membrane? And
further, if a membrane bone is found in the position ordinarily occupied
by a cartilage bone, is it to be regarded merely as the analogue and not
as the homologue of the latter?" (p. 296).

We may note here that many comparative anatomists of the period were
quite ready to decide Huxley's last question in a sense favourable to
the older, purely anatomical, view of homology. Owen, for instance, held
that difference of development did not disturb homologies established by
form and connections. "Parts are homologous," he writes, "in the sense
in which the term is used in this work, which are not always similarly
developed: thus the 'pars occipitalis stricte dicta,' etc., of
Soemmering is the special homologue of the supraoccipital bone of the
cod, although it is developed out of pre-existing cartilage in the fish
and out of aponeurotic membrane in the human subject."[236] Similarly he
pointed to the diversities of development of the vertebral centrum in
the different vertebrate classes as proof that devel