her
forms. Rather "do the _Holocephala_, _Plagiostomata_, and _Cyclostomata_
appear to us to be lower developmental stages individually
differentiated, so that the other fully differentiated Vertebrates
cannot easily be referred directly to their type" (p. 152, 1838). The
skull of these lower fishes is itself a specialised one; it is an
individualised modification of a simple type of skull. And this holds
good in general of the skulls of the lower Vertebrates--they are
individualised exemplars of a simple general type, not merely unmodified
embryonic stages of the greatly differentiated skulls of the higher
Vertebrates (p. 250, 1838). Differentiation within the vertebrate phylum
is therefore not uniserial, but takes place in several directions.
Reichert describes two sorts of modifications of the typical
skull--class modifications and functional modifications. The causes of
the modifications which characterise classificatory groups are unknown;
the second class of modifications occur in response to adaptational
requirements.

Reichert's two papers are of considerable importance, and Mueller's
remark in his review[209] of them is on the whole justified. "These
praiseworthy investigations supply from the realm of embryology new and
welcome foundations for comparative anatomy" (p. clxxxvii.).

The development of the skull was, however, more thoroughly worked out by
Rathke, and with less theoretical bias, in his classical paper on the
adder.[210] This memoir of Rathke's is an exhaustive one and deals with
the development of all the principal organ-systems, but particularly of
the skeletal and vascular. He confirmed in its essentials Reichert's
account of the metamorphoses of the first two visceral arches,
describing how the rudiment of the skeleton of the first arch appears as
a forked process of the cranial basis, the upper prong developing into
the palatine and pterygoid, the lower forming Meckel's cartilage, while
the quadrate develops from the angle of the fork. The actual bone of the
upper jaw (maxillary) develops outside and separate from the
palato-pterygoid bar. The cartilaginous rod supporting the second
visceral arch divides into three pieces on each side, of which the lower
two form the hyoid, the uppermost the columella. Like Reichert he held
the visceral arches to be parts of the visceral plates, containing,
however, elements from all three germ-layers--the serous, mucous, and
vessel layers.

The first gill