500 JULIA B.) PLATT: 
in Petromyzon, I believe to be homologous with those I have 
described in Necturus. 
The more dorsal of the two fusions, however, which v. Wijhe 
and Misses Johnson and Sheldon mention, are not homologous 
with those found on the dorso-lateral line in Necturus ; for, as 
fig. 1 shows, the primitive sense-organs connected with the 
glossopharyngeal and vagus ganglia lie in the epibranchial 
line, while in those segments immediately posterior to the ear 
the primitive dorso-lateral ridge is entirely reduced by the 
formation of mesectoderm, i. e. the vagus and glosso-pharyngeal 
ganglia with associated connective tissue, and can consequently 
not be directly associated with the formation of the lateral 
line. The same conclusions hold in regard to the facial 
segments. 
The section given in Pl. 36, fig. 5, chances to pass through 
several cells (a, a’, b,c) in the act of dividing; and as these 
cells surely take part in the formation of the nervous system, 
it is worthy of note that their planes of division lie in each of 
the three dimensions of space. We consequently have here 
not merely presumptive but positive evidence against Mall’s 
(27) statement that “the primitive growing point of all ver- 
tebrate nerves is in the layer of cells on the outermost side of 
the ectoderm, and the axis of division is parallel with the 
ectoderm.” Neither is the primitive growing point of the 
lateral line nerve in the layer of cells on the outermost side of 
the ectoderm, nor is the axis of division always parallel to the 
surface of the ectoderm. Yet Mall studied Necturus ! 
The series of sections (Pl. 36, figs. 2—5) serve also to de- 
monstrate the inaccuracy of Beard’s (4) statement that the 
ganglion splits off from the deeper layers of the ectoderm, 
leaving an external sense-organ. This does not happen in 
Necturus. The large dorso-lateral and epibranchial ganglia 
are formed from cells which split off en masse, leaving the 
ectoderm external to them for the time thin. A sensory ridge 
may appear later in the exact place where the ganglion arose, 
as happens in the supra-orbital line, or sense-organs may form 
at either side of the ganglion Anlage, as in the vagus region, 
