488 JULIA B. PLATT. 
Beginning at the anterior extremity of the dorsal longitudinal 
ridge, we find that it gives rise to cells which join the trige- 
minal portion of the neural crest, and take part in the formation 
of the Gasserian ganglion and its anterior continuation—the 
ramus ophthalmicus profundus. This nerve is formed from the 
ectoderm in the same manner as are the cranial ganglia. 
The migration of cells into the trigeminal mesectoderm 
extends from the anterior limit of the dorsal ridge to the inter- 
segment between the mandibular head cavity and the anterior 
of the two somites which in Necturus as in Scyllium (van 
Wijhe, 37) lie above the hyomandibular cleft. In this inter- 
segment the proliferation of mesectoderm extends downwards 
to the epibranchial ridge, which here passes over the hyoman- 
dibular cleft, and curves towards the oral invagination in 
conformity to the cranial flexure. 
In the continuous band of deep ectoderm above the eye, 1 
have been unable to distinguish any limited area, which in 
giving rise to ganglion cells on the profundus would be the 
homologue of what is described by Beard (8) as the “ sense- 
organ” of the ciliary ganglion in the Elasmobranchs. 
This area, which is distinct in Elasmobranchs, appears in 
Necturus to have fused with the dorso-lateral thickening of the 
ectoderm connected with the Gasserian ganglion, so that we 
here have but one band of deep ectoderm, which proliferates cells 
continuously to the Gasserian ganglion and to the profundus. 
I do not find that the band of deep ectoderm above the eye 
disappears at any time throughout its entire length, yet it 
becomes greatly reduced in width by the migration of cells 
into the trigeminal mesectoderm, and shortly previous to the 
stage with which this paper begins the band is interrupted by 
spaces, from which the cells have migrated so rapidly as to 
leave the ectoderm above scarcely, if any, deeper than on the sur- 
rounding surface of the head. This condition is but transitory, 
and other cells soon occupy the place of those that have gone, 
restoring the depth and continuity of the ridge. 
Anterior to the hyomandibular cleft the lens arises, as 
described by von Kupffer (23) in Petromyzon as a specialisation 
