DIFFERENTIATIONS OF ECTODERM IN NEOCTURUS. 541 
(2) The supra-orbital line of sense-organs and the ear form 
by direct modification of the primitive ridge of the dorso- 
lateral line. Sense-organs connected with the dorsal branches 
of the glossopharyngeus and vagus form from ridges that 
secondarily grow upwards from the epibranchial ridge after 
the primitive ridges of the corresponding intersegments have 
dis..ppeared in giving rise to mesectoderm. The infra-orbital 
line of sense-organs, the otic sense-organ, the lower organs 
supplied by the glossopharyngeal nerve, and the organs of the 
epibranchial vagus line develop by the modification of the primi- 
tive epibranchial ridge. The primitive intersegmental thickening 
of the ectoderm where it touches the hyomandibular pocket in 
the line of the gill cleft is directly modified into the hyoman- 
dibular line of sense-organs. The mandibular line develops as a 
secondary outgrowth from the hyomandibular line. The primi- 
tive ridge of ectoderm at the third vagus cleft is the beginning 
from which the ventral trunk line develops. The anterior part 
of the primitive ventral longitudinal ridge is retained in the 
ventral part of the hyomandibular line of sense-organs (the 
internal mandibular). The lateral lines of the trunk grow 
from the head backwards through indifferent ectoderm, and 
do not result from the direct modification of primitive ridges. 
(3) The primitive ridges differ functionally as well as struc- 
turally from those of the lateral line. The former are the 
source of both nervous and connective tissues, the latter of 
nervous tissues alone. 
(4) The primitive ridges of the trunk, like those of the head, 
are sources of addition to the mesectoderm. 
(5) Few of the cells that disappear from the secondary 
sensory ridges of the trunk, as the sense-organs arise, enter 
the lateral nerve. The majority of these cells migrate betweeu 
the two layers of the ectoderm towards the terminal point of 
growth in the advancing line. 
(6) The third vagus cleft forms in the intersegmental plane 
once occupied by the second vagus cleft, and the primitive 
vagus myotome divides in the plane secondarily occupied by 
the second vagus cleft, while the nerves of the second vagus 
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