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The Development of the Cartilaginous Skull ete. in Necturus. 427 
two parts. In the ventral part muscle fibres begin to form, while 
in the dorsal part procartilage arises. This plate of procartilage lies 
primarily ventral to the floor of the ear, but is carried laterally 
with the development of the ear, and as cartilage forms, comes to 
lie below the floor of the external semicircular canal. This later 
position of the plate is shown in fig. 25, where the ground shade also 
indicates that from the plate as a centre of chondrification, cartilage 
is gradually extending dorsalwards. 
In Necturus the tissue occupying the space between the floor 
of the capsule and the basal plate becomes gradually converted into 
cartilage in such manner that one cannot speak of this cartilage as 
properly an outgrowth either from the parachordal or periotic cartilage. 
The uniting cartilage includes in its- formation indifferent cells of 
the mesenchyma lying between the basal plate and the auditory 
capsules, which give rise to cartilage without passing through the 
procartilage stage. A similar direct convertion of cells of the 
mesenchyma into cartilage in this region in Triton has been noted 
by Srour (80). 
The skull in Necturus 19 mm long. 
Figs. 15 and 16, pl. XVII, represent a model of the cartilaginous 
skull in an embryo of 19 mm, in which the cartilaginous elements 
of the head are well established. In fig. 15, the skull is viewed 
from the ventral surface, showing chiefly the branchial bars, which, 
with the exception of those of the mandibular arch, have been omitted 
in fig. 16, where one views the chondrocranium from the side and 
from above.: Merely a closer grouping of cells indicates the position 
of the internasal plate which later connects the trabecular bars 
anteriorly; and the antorbital processes, although outlined in the 
figures, are as yet unchondrified, still consisting of the closely grouped 
nuclei that characterize procartilage — here mesectodermic. 
The trabecular bars are round or oval in section, thus resembling 
the primitive form of these bars in the Anura more closely than 
in Triton or Siredon, where according to Sréur ('80) the trabe- 
eulae are from the first nearly vertical plates at the side of the 
brain. From fig.17 it is seen that the trabecular bar, a short distance 
posterior to the antorbital process, is connected in three places with 
a short dorsal bar of cartilage lying parallel to the trabecula, 
thus leaving two spaces surrounded by cartilage. Through the anterior 
of these openings the optic nerve passes outwards. Through the 
