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 reduced in size, as compared with the 

 other regions of the brain. 



In the Selachii the thalamencephalon is distinctly separated from 

 the mesencephalon (Fig. 281, d), but in many Teleostei it is intimately 

 connected with it. The anterior portion of its roof contains the 

 above-mentioned cleft, and this portion is not unfrequently de- 

 veloped into an elongated tract, which forms a longitudinal com- 

 missure between it, and the prosencephalon (many Sharks and 

 Ganoids). The remainder of the primitive roof, which contains the 

 hinder portion of the cleft, is sometimes very large, and divided 

 into two hemispheres; this is the case in the Selachii and many 

 Teleostei. The floor of this segment, which surrounds the infun- 

 dibulum, and forms the lobi inferiores at the base of the brain, is 



Fig. 281. Brain of a 

 Shark (Scyllium catn- 

 lus). h Olfactory lobes. 

 g Prosencephalon. d 

 Thalam- and mes-ence- 

 phalon. b Metencepha- 

 lon. a Myelencephalon, 

 Nasal capsules (after 

 Busch). 



