SKULL. 479 
slits and support the pharynx; above Amphibians, they are 
less completely developed. 
In the Elasmobranch fishes, the mandibular and hyoid arches do not 
form any direct part of the cartilaginous brain-case, but in the Tele- 
osteans and thence onwards, the cartilages or bones arising in connection 
with the mandibular and upper part of the hyoid arches contribute 
directly to the formation of the skull. The hyoid proper,. or lower 
part of the hyoid arch, forms the skeleton supporting the' tongue. 
Cartilages arising in the lower part of the third visceral arch assist 
in the formation of the hyoid bones of the higher Vertebrates, and parts 
of two other arches appear to help in forming the laryngeal skeleton of 
Mammals, 
The mandibular arch in Elasmobranchs and frogs divides into a lower 
portion—Meckel’s cartilage—which forms the lower jaw or its basis, 
while from the upper portion a bud grows forward, the palato-pterygo- 
quadrate cartilage, which forms the upper jaw in shark and skate, and 
has a closer union with the skull in the frog. In higher Vertebrates 
the lower portion of the mandibular always forms the basis of the lower 
jaw, a quadrate element is segmented off from the upper part, but the 
palato-pterygoid part seems to arise more independently. The hyoid 
arch also divides into a lower portion, the hyoid proper, and an upper 
portion, the hyo-mandibular, which may connect the jaws with the skull, 
or from Amphibians onwards may be more remarkably displaced and 
modified as a columella or stapes connected with the ear. 
Returning now to the brain-box itself, we must notice 
another complication,—the development of ‘ membrane” 
bones. If we examine the skull of the skate, we find that 
the brain lies within a cartilaginous capsule ; but this is not 
entirely closed, spaces (the ‘fontanelles) being left in the 
roof, which during life are covered only by the tough skin 
with its numerous “dermal denticles.” In the sturgeon, 
again, the small skin-teeth are replaced by stout bony plates 
covering over the cartilaginous capsule. From such super- 
ficial bony plates it is supposed that the “membrane” 
bones, or ossifications in membrane, which form so import- 
ant an element in the skull of the higher Vertebrate, have 
originated. 
In some bony fishes, notably the salmon, we find the brain enclosed 
in a double capsule. Inside there is a cartilaginous brain-case in which 
what are called centres of ossification have appeared, and upon this a 
layer of membrane bones is placed, which can be readily removed with- 
out injury to the cartilage beneath. In general, however, we must 
recognise that, with the appearance of membrane bones, two changes 
tend to occur,—first, the cartilaginous cranium tends to be reduced and 
to exhibit considerable openings; second, in the remaining cartilage 
centres of ossification appear, and we thus have ‘‘ cartilage” bones 
