. Elements of Comparative Anatomy. 504 COMPAEATIVE ANATOMY. second cerebral vesicloj and gives rise to a diverticulum, whicli is found in all Craniota, and is known as the infundibulum. From tlie lower side of the head a depression of the ectoderm grows towards this diverticulum; later on, the ingrowth becoming pinched off, forms a portion of the cerebral appendix attached to the infundibulum (hypophysis). The range of the position of the depression for the hypophysis as far forward as the entrance into the cavity of the mouth enables us to recognise in this structure an organ, which primitive
. Elements of Comparative Anatomy. 504 COMPAEATIVE ANATOMY. second cerebral vesicloj and gives rise to a diverticulum, whicli is found in all Craniota, and is known as the infundibulum. From tlie lower side of the head a depression of the ectoderm grows towards this diverticulum; later on, the ingrowth becoming pinched off, forms a portion of the cerebral appendix attached to the infundibulum (hypophysis). The range of the position of the depression for the hypophysis as far forward as the entrance into the cavity of the mouth enables us to recognise in this structure an organ, which primitively did not belong to the nervous system at all, and the function of which is still a matter for speculation. Just as the upper wall between the fore- and twixt-brains gets thinned out, so too the roof of the myelencephalon is thinned out, in such a way that no roof remains but such as is formed by the outer- most vascular layer of the nerve-centre, the pia mater. The large cavity which is thus roofed over forms the fourth ventricle. The ventricles, or cavities in the portions derived from the primary cerebral vesicles, are connected with one another in just the same way as the cavities of the cerebral vesicles. The brain of the Cyclostomata is the simplest in form; among them the lowest grade is occupied by the Myxinoidea, where the various segments have very nearly the same characters. A portion, which is developed from the fore-brain, and which gives off the olfactory nerves (bulbus or lobus olfactorius), generally forms large lobes, which, in the Selachii, are connected with the brain by a more or less long tractus olfactorius (Fig. 281, h). The ventricle of the prosencephalon is continued into them. They may also be fused with the prosencephalon, which is larger than the other divisions, in the Selachii ((/), and gives indica- tions of a separation into two, four, or more paired pieces. _ It is large also in the Ganoidei (Fig. 282, g), while in many Teleostei it is greatly
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