4.94 JULIA B. PLATT. 
clefts arise successively in the second intersegment. The poste- 
rior continuation of the ventral ridge on the trunk of the embryo 
has disappeared with the other longitudinal ridges of the trunk. 
Of the transverse ectodermic ridges at the gill clefts but two 
remain, that at the posterior margin of the last cleft and the 
hyomandibular ridge. The other ridges were lost as the clefts 
formed, but although the ectodermic ridge at the hyomandi- 
bular cleft touched the wall of a corresponding pocket from 
the alimentary canal no cleft formed, and the endoderm after- 
wards receded, leaving the ridge of ectoderm in the position it 
originally occupied. This ridge of deep ectoderm, once the 
rudiment of the hyomandibular cleft, is now the Anlage of 
the hyomandibular line of sense-organs. I prefer with Ewart 
(10) the designation ‘‘ hyomandibular ” for this line of sense- 
organs, rather than “ operculo-mandibular,” which Allis em- 
ployed to designate an homologous branch of the lateral line in 
Amia (1). 
In fig. 1 a short ridge is seen to extend forward from the 
middle of the hyomandibular ridge. This is the beginning of 
the mandibular line of sense-organs, which thus branches off 
from the hyomandibular line and grows towards a point near 
the mid-ventral axis of the body, posterior to the oral invagi- 
nation, where it fuses with the ventral longitudinal ridge. It 
is worthy of note that while two primitive intersegmental 
ridges meet above the hyomandibular cleft, below, two sensory 
lines form from that cleft. 
Pl. 36, fig. 2,a@, is a section through the supra-orbital ridge 
of an embryo at the stage of development represented in fig. 1, 
where the plane of the section is indicated. The cells of the 
supra-orbital ridge are seen to have the radial arrangement 
peculiar to the sense-organs of the lateral line. The section, 
however, does not cut a sense-organ. The cells of the sensory 
ridge arrange themselves radially about the long axis of the 
ridge before dividing into groups arranged about a point above 
the centre of a sense-organ. In cross-section, therefore, the 
sensory ridge presents the same appearance as the median plane 
of a sense-organ. 
