The Development of the Cartilaginous Skull ete. in Necturus. 451 
at the side of the chorda, adjoining anteriorly the muscle fibres formed 
in the ventral part of the third postotic somite. As the basal plate 
develops and the bases of the praeoccipital and occipital arches fuse 
into its continuous cartilage, the ventral fibres of the second and 
third postotic somites atrophy, and the ventral fibres of the fourth 
somite become attached anteriorly to the base of the occipital arch 
external to the condyles. The muscle formed by the union of the 
dorsal parts of the second and third somites persists, however, and 
extends forwards above the auditory capsule. Thus each of the 
primitive myotomes in Necturus is represented in the muscular tissue 
of the animal 46 mm long, and doubtless of the adult. 
Sometime after the union of the dorsal part of the second and 
third postotic somites, the myoseptum dividing the third postotic 
somite from the fourth, ceases completely to separate these somites, 
and it is found that in the median dorsal region, above the spinal 
column, a pair of long muscle bands extend from the first neural 
arch, passing the myoseptum between the third and fourth postotic 
Somites without division, to the posterior surface of the muscle 
temporalis. Mivartr says (69, page 453) that the temporalis 
muscle »arises from the middle of the upper surface of the skull, 
and (by fascia) from the first neural spine«. The connection of the 
temporalis with the first neural spine is accomplished, in Necturus 
of 46mm, by means of the muscle bands above described, which 
are composed of fibres from the median dorsal part of the second, 
third, and fourth postotic somites. 
In figs. 23 and 24, two enlargements of the coelom, connected 
with the development of the pronephros are seen below the fifth and 
sixth postotic somites, while from the fifth, fourth, third, and possibly 
from the second somite, strands of cells are seen to extend ventral- 
wards, uniting in a triangular group of mesoderm cells. These cell 
strands are from buds growing ventralwards from the somites above. 
Longitudinal sections show the relations indistinctly since the growing 
buds bend outwards. I have therefore combined the results derived 
from the study of a number of series of sections in the diagram 
shown in fig. 25. 
At the right of the figure (25), the auditory vesicle is repre- 
sented, to the left of which, the line of myotomes begins with the 
second postotic segment. In this segment lies the first vagus root, 
The muscle fibres of the dorsal part of the second postotic segment 
are represented as fused with those of the dorsal part of the third 
30* 
