274 Journal of Comparative- Neurology and Psychology. 



s. s. — somatic sensory column. 

 s. V. — secondary vagus tract. 

 sy. g. — sympathetic ganglion. 



T — hypothetical center for the nervus thalamicus (visceral). 

 t. b-t. — tractus bulbo-tectalis. 

 /. l-b. — tractus lobo-bulbaris. 

 /. t-b. — tractus tecto-bulbaris. 

 t. t.l. — tractus tecto-lobaris. 

 /. o. — tractus opticus. 

 t. o-h. — tractus olfacto-habenularis. 

 /. .r.///. —tractus sacco-thalamicus. 

 tr. n. — trunk nerve. 



An arrow point in Fig. 8 indicates the hypothetical margin of the primitive 

 neural plate; in Fig. lo indicates the anterior end of the brain. 



PLATE I. 



Figures i-y illustrate the longitudinal zones of the brain and the relation of 

 the brain commissures to them. Fig. 7 is a median sagittal section of the brain 

 of a fish on which are projected the zones of the right half of the brain. The 

 location of the various decussations is also indicated by symbols which are ex- 

 plained on the plate. The places of exit of the nerve roots are indicated by the 

 usual Roman numerals. The lateral line roots arise near to or above VIIL The 

 olfactory centers have been shown in the same shading as the visceral sensory 

 centers in the medulla oblongata. The secondary vagus nucleus and the hypo- 

 thetical nucleus of the nervus thalamicus are also shown in the same way. The 

 ventro-mesial zone, marked by small circles in this and Fig. i, is made up of cells 

 which closely surround the mid-ventral furrow of the ventricle, "hese cells form 

 important nuclei in the inferior lobes, the nucleus thaeniae, corpus inter- 



pedunculare and nucleus of Meynert's bundle (and possibly the lower olive in 

 fishes). This area corresponds to or includes the Bodenplatte distinguished by 

 His in the embryonic brain. Fig's. 1-6 represent cross sections at the levels indi- 

 cated by the reference lines. More structures are shown in some of the fi-gures 

 than would appear in an actual section. 



Fig. I. — Section through the superior commissure, ganglia habenulae, 

 thalamic nucleus of the somatic motor fasciculus, and the inferior lobes in front of 

 the saccus vasculosus. The position of several fiber tracts is shown. 



Fig. 2. — Through the tectum opticum, dorsal decussation, ansulate com- 

 missure, and the nucleus of N. III. 



Fig. J. — Through the cerebellum and the root of N. V. Both somatic and 

 visceral decussations in the cerebellum are shown. 



Fig. 4. — Through the root of N. IX. The difference in the course of the 

 secondary tracts from the somatic sensory and the visceral sensory centers is 

 shown. 



Fig. 5. — Through the commissura infima Ham.eri. 



Fig. 6. — Through the root of a spinal nerve and showing the dorsal and 

 ventral decussations of the spinal cord. 



