350 



CHORDATE ANATOMY 



Instead of only two brain divisions, cyclostomes have five, telencephalon, 

 diencephalon, mesencephalon, metencephalon, and myelencephalon. 



Ontogenesis reveals, however, that these five vesicles of cyclostomes and 

 higher vertebrates develop from the original three which are correlated 

 with the three major senses, smell, sight, and hearing. The primitive 

 fore-brain, which in the opinion of most morphologists corresponds to 



TELEN- 



MYELENCEPHAUON 



epiphysis' 

 parietal organ 

 habenular ganglion 

 dorsal sac' 

 paraphysis 

 Monroe's foramen 



HEMISPHEREv 



OLFACTORY /^ 

 LOBE 



BASAL ganglion' 

 ANT COMMISSURE 

 CHORIOID PLEXUS' 



.POST. COMMISSURE 

 /TECTUM OPTICUM 



SPINAL 

 [GANGLION 



POST RECESS 

 INFUNDIBULUM 

 \ HYPOPHYSIS 

 ^OPTIC THALAMUS 



LATERAL VENTRICLE, 

 BASAL GANGLION 

 HEMISPHEREi 



OPTIC THALAMUS 

 EPIPHYSIS 

 VENT IE I I /HABENULAR GANG. 



ANT. COMMISSURE' 



'VELUM TRANS 

 PARAPHYSIS 



CHORIOIDEA 

 RHOMBOIDAL FOSSA 



Fig. 310. — Diagrams of the vertebrate brain, based upon the brain of a cyclostome. 

 A shows the brain in median longitudinal section, with nerves as if projected upon 

 the median plane. B is the brain viewed from above. (Redrawn from Plate, after 

 Biit.schli.) 



the prosencephalon of Amphioxus, becomes the telencephalon and dien- 

 cephalon, the mid-brain continuing as the mesencephalon, and the original 

 hind-brain divides into metencephalon and myelencephalon. Since the 

 cyclostome brain may be taken as the complete prototype of that of all 

 vertebrates, and since most of its features persist in higher forms, these 

 are worthy of mention in some detail. (Fig. 309) 



The telencephalon is paired, in correlation with the development of 

 paired olfactory lobes. Although paired eyes develop from the fore-brain, 



