492 JULIA B. PLATT. 
of these two intersegments has disappeared entirely in giving 
rise to part of the facial mesectoderm. 
The ear has developed from the dorso-lateral thickening in 
the hyobranchial intersegment, and it will be noticed that 
although the ear undoubtedly belongs in the lateral line 
system, and is in fact the centre of that system, it is not 
properly a “ branchial sense-organ,” as Beard suggests (2), for 
this term cannot be accurately applied to sense-organs above 
the epibranchial line. 
The dorso-lateral thickening of the following intersegment 
has for the time disappeared in giving rise to cells that now 
form part of the vagus mesectoderm. In the second inter- 
segment posterior to the ear a knob-like structure at the ante- 
rior end of a longitudinal club-shaped ridge is the beginning 
of the most dorsal of the lateral lines of the trunk. Posterior 
to this intersegment the primitive dorso-lateral ridge has dis- 
appeared, and anterior to the intersegment, as has just been 
pointed out, two of the primitive intersegmental differentia- 
tions in the dorso-lateral line are missing, namely, those 
connected originally with the facial and glossopharyngeal 
ganglia. 
It is evident that in following the later distribution of sense- 
organs one must look to the most dorsal of the lateral lines of 
the trunk for representation or omission of sensory differentia- 
tions in intersegmental lines posterior to the first intersegment 
back of the ear. 
At its anterior extremity the ridge of the epibranchial line, 
about to give rise to the infra-orbital sense-organs, blends with 
the wide and deep nasal epithelium which has now begun to 
invaginate. Following the line upward, one finds that at the 
first of the intersegments meeting in the hyomandibular cleft 
the ridge fuses with that of the dorso-lateral or supra-orbital 
line, while above the hyomandibular cleft a round area of 
sensory epithelium has been cut off from the infra-orbital 
ridge as the primitive epibranchial sense-organ of the hyo- 
mandibular cleft. 
The epibranchial thickening in the posterior of the two 
