336 The American Naturalist. ‘[April, 
latter is to be regarded as an organ of special sense. The subse- 
quent development of the ganglionic system is entirely independ- _ 
ent ofthe special sense organs. Though later the ganglia in the — 
region of the head are directly connected with the branchial sense _ 
organs, yet the former are never developed at the expense of the _ 
latter, Beard, Froriep, and Spencer to the contrary. The olfactory — 
ganglia are probably formed from the neural ridge. They are not 
derived from the cells of the nasal fossa. The posterior roots of 
the cranial and spinal nerves are at first cellular, and are formed “ 
from that part of the neural ridge placed between the dorsal — 
borders of the medullary tube. : 
Houssay (’90), in his observations on the development of the 
Axolotl, agrees with Beard as to the ectodermal origin of the — 
Anlagen of the dorsal nerve-roots. The cranial ganglia first 
appear as an unsegmented ectodermal band, which afterwards 
extends into the trunk, forming the lateral line and nerve. Inthe — | 
meantime, while this posterior differentiation is occurring, the 
band anteriorly segments to form the cranial ganglia. The cen- — 
tral nervous system, though at first unsegmented, is soon meta- 
meric, both in brain and spinal cord, and this metamerism is _ 
called “neurotomy.” The dorsal nerve-roots arise each behind 
the “neurotome” of its segment,’ this relation being secondary. 
The author thinks there is a complete homodynamy of the periph- : 
eral nervous system in all the metameres of the body. Indis 
cussing the metamerism of the head he states that the segments 
do not appear with any regularity as to time and location. The 
neurotomes, neuromeres, branchiomeres, and myotomes agree © 
the manner of segmentation. He believes he finds evidence 
the existence of an oculohypophysial, a buccal, a hyomane 
lar, and an auditory segment. The IV. and VI. nerves cannot ! 
certainly identified as ventral roots. ag 
Gaskell (89a, ’89é, 90) considers the central nervous sys 
of vertebrates as made up of two parts: a non-neryous § 
ing tube, and a nervous portion surrounding that tube. He ba 2 
his observations on Ammoccetes, and concludes that the 1 T 
nervous tube is the altered alimentary canal of a Crustacea 
3 Vide Platt and McClure. 
