. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology; Zoology. SAUGENT: THE OrTIC KEFLEX APPARATUS OF VEKTEDUATKS. 151 region of the vagus lobes has a greater width than any other portion of the brain except the olfactory lobes. The sinus rhomboidalis is a wide open space covered by the greatly convoluted and vascular plexus chorioideus III (Fig. A). The cerebellum is but slightly developed as an independent structure. The optic lobes are large and nearly spheri- cal. Along the median plane they are partly separated from each other by a slit-like sinus covered by the gre
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology; Zoology. SAUGENT: THE OrTIC KEFLEX APPARATUS OF VEKTEDUATKS. 151 region of the vagus lobes has a greater width than any other portion of the brain except the olfactory lobes. The sinus rhomboidalis is a wide open space covered by the greatly convoluted and vascular plexus chorioideus III (Fig. A). The cerebellum is but slightly developed as an independent structure. The optic lobes are large and nearly spheri- cal. Along the median plane they are partly separated from each other by a slit-like sinus covered by the greatly convoluted plexus chorioideus XL The diencephalon is well developed and more conspicuous than in any other vertebrate brain. It consists of the two lateral elongated lobes of the optic thalamus separated dorsally by an open sinus, which is covered by the plexus chorioideus I, an almost simple membrane. The right ganglion habenula is an almond-shaped lobe, so greatly devel-. FiouRE A. Petromyzon marimis, adult. Nearly median parasagittal section of the brain, showing Reissner's fibre projected on the plane of section. For meaning of abbreviations, see Explanation of Plates, p. 257. oped as to lie mostly in the median plane (Plate 1, Fig. 4). The left ganglion habenula is much smaller and lies on the left side, partly cov- ered by the right ganglion. The prosenceplialon consists principally of two enormous olfactory lobes. The cyclostome brain has the lateral halves of its roof less completely connected by nervous structures than that of any other vertebrate. A sagittal section of the brain shows the roof largely vascular or mem- branous. The only nervous structures in the median plane are the small and primitive cerebellum, the posterior portion of the mesencei)ha- lon, the posterior commissure, and the right ganglion habenula. The essentially non-nervous character of the median brain-roof and its al- most complete division into bilateral halves are the conspicuous features
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