924 THE AMERICAN NATURALIST.  [Vor. XXXIV. 
the dorsal or ganglionic root, internally by the lateral angle 
of the central cavity, shown in the section. In the ventral 
zone the development is more advanced, — the three primary 
layers in the walls of the neuraxis (or medullary tube) being 
clearly differentiated, namely: the light outer neuroglia layer; 
the middle layer, commonly called the mantle layer, but which 
might be better named the neurone layer; and, finally, the inner- 
most or ependymal layer. These three layers are primary and 
appear throughout the entire brain and spinal cord. The sepa- 
ration of the dorsal and ventral zones is of fundamental mor- 
phological importance, and their characteristics must be clearly 
understood before the anatomy of the brain is studied. The 
ventral zone contains all the neurones from which fibres arise 
forming the ventral nerve root (V.R.). Following downwards 
in the figure we come to the section of the jugular vein (/ug.), 
just inside of which lies the common trunk (70.77) of the united 
vagus and accessorius nerves, and also the lower part of the 
glossopharyngeal ganglion (9) Lower down and nearer the 
ectoderm lies the facial nerve (7), and again still lower is 
the large trunk of the inferior maxillary (J/v.7.), or mandibular 
branch of the trigeminal nerve. Between the two nerves last 
mentioned is a long oblique slit, which is found at another 
level to communicate with the pharynx (P7.); this slit is the 
inner or entodermal portion of the first or auditory gill-cleft, 
and is the anlage of both Eustachian tube and of the cavity of 
the tympanum. Its outer (in Fig. 4, upper) end lies near the 
bottom of an ectodermal groove; this groove (Fig. 1, Au.) 
divides the mandibular process from the hyoid arch, and is the 
anlage of the meatus auditorius externus. The hyoid arch is 
marked in Fig. 4 by the facial nerve (7) and the arched over- 
lying ectoderm. The internal carotid artery (C.7.) appears near 
the optic nerve (Of). External to this artery lies the superior 
maxillary nerve, which, however, does not show in the figure. 
The vesicular lens (Z.) and the cup-shaped retina (Ret.) of the 
eye can be readily identified. The fore-brain has begun to 
form two lateral expansions, the cavities of which ultimately 
become the lateral ventricles (Z. V.) of the brain, and the walls 
of which are the anlages of the hemispheres (/.). 
