DIFFERENTIATIONS OF ECTODERM IN NEOTURUS. 525 
a nearly horizontal position, passing through the mesectoderm, 
with the cartilage Anlage above and the branchial wall below, 
as seen in fig. 23, Pl. 36. This figure shows the distal part of 
the glossopharyngeal nerve. Near the median plane of the 
embryo the nerve ends in small branches. Fig. 21 does not 
represent the distal part of these glossopharyngeal and vagus 
nerves. 
The lateral branchial nerves of the glossopharyngeus and 
vagus, which run backwards and outwards, and which appear 
to be serially homologous with the ramus hyoideus facialis, 
are chiefly distributed to the branchial muscles which lie 
against the anterior wall of the posterior cleft, although 
branches also go to the skin or are lost on the walls of the 
blood-vessels. As the hyoid muscle is very large, and fills the 
posterior half of the hyoid arch, its nerve is correspondingly 
large, and to reach the muscle measures but half the width of 
the arch, while the following smaller homologous nerves reach 
their respective muscles near the posterior wall of the arch. 
The branchial nerves of the second vagus arch arise at this 
stage from the root of the nerve of the first arch, and only 
later do these nerves acquire independent connection with the 
ganglion. The course of the nerves in the posterior arch is 
similar to that of those in the first vagus arch. One nerve 
passes downwards against the posterior wall of the branchial 
cleft, the other passes into the posterior arch supplying its 
lateral musculature and the external gill. The secondarily 
acquired independence of the nerves of the posterior vagus 
arch seem to me significant in connection with the manner in 
which the third vagus cleft forms and the primitive vagus 
myotome divides. 
Two small branches, that enter the ganglion by a common 
root, supply the two divisions of the glossopharyngeal sensory 
ridge. The nerves connecting the following intersegmental 
(vagus commissural) and epibranchial ridges with the vagus 
ganglion appear to enter the ganglion at this stage by separate 
roots. I may be mistaken in this, however, for in an embryo 
but little older the nerves can be traced to a common stem, 
