506 JULIA B. PLATT, 
facial ganglia. The first protovertebra posterior to the ear lay 
below the Anlage of the glossopharyngeal ganglion, but gives 
rise to no myotome, being apparently crushed out of existence 
by the growth of the ear. The second segment posterior to 
the ear gives rise to the first myotome, and contains the vagus 
ganglion. The third segment is that of the first spinal 
ganglion, &c. 
The two anterior spinal ganglia possess no dorsal root, but 
consist each of a small group of cells at the base of the motor 
nerve. It was of interest to know from what source these 
ganglia came, and with the solution of this problem in view I 
turn to earlier stages in the development of the peripheral 
nervous system of the trunk. 
The posterior division of the neural crest begins with the 
facial Anlage, and from it successively the neural portions of 
the facial, glossopharyngeal, and vagus mesectoderm are 
separated. The facial and glossopharyngeal portions of the 
crest lie above protovertebre that develop no myotomes, but 
immediately disappear in giving rise to mesenchyme over 
which the cells of the neural Anlage migrate beneath the skin 
to the epibranchial ridge. 
The vagus segment, however, develops its proper myotome, 
and when the cells of the neural crest migrate downwards only 
the anterior part passes over the dorsal wall of the proto- 
vertebra. The posterior part of the Anlage lies, as in the 
spinal region, between the brain and the protovertebra. When 
the protovertebra begins to extend dorsally as its muscle-plate 
forms, the growth of its anterior part is checked by the vagus 
Anlage, which passes over the protovertebra from the brain to 
the epibranchial ridge. Therefore only the posterior part of 
the myotome can grow upwards. This it does, and then ex- 
tending forwards replaces the missing dorsal part of the an- 
terior half of the myotome. In consequence the anterior part 
of the vagus ganglion appears at this stage of development to 
cut half through the myotome. 
Pl. 36, figs. 6 and 7, illustrate this relation. Fig. 6 is 
anterior to fig. 7 by one third of the width of a myotome, and 
