. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates. BRAIN 201 embryonic epithelial character: this is usually regarded as being- due to regressive metamorphosis, the cause of which, however, is difficult to explain. The relative distribution of the gray and white matter differs in various pares of the brain. Connecting the two lateral halves of the brain are certain transverse bands of nerve-fibres or commissures. In addition to a small superior or habenular commissure in the pallium (Fig. 150), an anterior commissure is present in the posterior region of the secondary for
. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates. BRAIN 201 embryonic epithelial character: this is usually regarded as being- due to regressive metamorphosis, the cause of which, however, is difficult to explain. The relative distribution of the gray and white matter differs in various pares of the brain. Connecting the two lateral halves of the brain are certain transverse bands of nerve-fibres or commissures. In addition to a small superior or habenular commissure in the pallium (Fig. 150), an anterior commissure is present in the posterior region of the secondary fore-brain, a middle in the primary fore-brain (in Mammals only), and a posterior in the anterior part of the mid- brain. In addition to these, others may be developed in the pallial region ( anterior and posterior pallial commissures, cf. Fig. 165); and amongst Mammals those known as the corpus callosum and fornix are of great importance. Mesencephalon Diencephalon Pineal-undPftrietal- / organ, ,'Zirb Veuan. tmnsversam 'piihelMe QuErlhlL. Fit;. 150.—DIAGRAMMATIC LONGITUDINAL SECTION THROUGH PART OF THE EMBRYONIC BRAIN. Zirbefpolster, pineal cushion. The outer surface of the hemispheres is more or less smooth, except amongst the Mammalia, in which fissures (sulci) and convolu- tions (gyri) may be present. These consist of folds of the entire pallium or cortex, and they cause a greater or less increase of the superficial area. From the primary fore-brain, the ventricle of which is walled in anteriorly by the lamina terminalis, the following structures also arise (Fig. 149):—the optic thai ami, formed as thickenings of its basal walls, and the ganglia- haJicntt/cc on the posterior lateral margin of the dorsal region, with the superior conuiiixxurc between them; the primary optic vesicles, arising as paired ventro-lateral outgrowths, from which the optic nerves and retina with its pigment epithelium are derived later; the pineal apparatus, developed as. Please note that these
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