498 JULIA B. PLATT. 
appears, leaving the skin where it now is apparently un- 
modified. 
There is in Necturus a qualitative as well as structural 
difference between the primitive and secondary ectodermic 
ridges. The primitive ridges, formed by the multiplication of 
layers in the ectoderm without radial rearrangement of the 
cells, may disappear, leaving no apparent modification in the 
structure of the embryo; or they may be directly modified into 
secondary ridges, as is the case with the primitive ridge of the 
hyomandibular cleft ; or, lastly, they may be the source of large 
additions to the mesectoderm, as in the dorso-lateral and 
epibranchial lines on the head. The mesectoderm thus pro- 
liferated may be either exclusively ganglionic, as when the 
lateral line ganglion is cut from a primitive epibranchial ridge ; 
or may be composed of elements partly ganglionic and partly 
connective tissue, as in the primitive dorso-lateral proliferation 
connected with the trigeminus; or, finally, the proliferation 
may be exclusively of connective-tissue cells, as at the margin 
of the gill clefts below the epibranchial line. 
Where the proliferation of mesectoderm is composed of both 
nervous and connective-tissue elements, it often appears impos- 
sible to decide whether cells not closely grouped in the body of 
a ganglion are to be classed with the nervous system, or are to 
form the basis of a branchial cartilage, until the prolongations 
of some of the scattered and apparently homogeneous mesec- 
toderm cells show the fine fibrillar striation peculiarly nervous. 
The secondary ridges, however, composed of cells radially 
arranged, are the source of neryous structures alone, 
and their radial arrangement may be co-ordinated with the 
fibrillar differentiation of nerve-cells forming part of the 
mesectoderm, as indicating an exclusively nervous character. 
In fig. 1 it will be seen that the chief nerves of the lateral 
line system are beginning to develop. From the vagus-glosso- 
pharyngeal group, fibres in a dorsal bundle lose themselves in 
the main lateral line of the trunk at the point where fibres of 
the dorsal lateral line, following the growth of the knob of cells 
at the head of the main line, will soon appear. Another bundle 
