VERTEBRATES. 



29 



known as ventricles or coelise, communicate with each other, and with the central canal 

 of the spinal cord. They are distinguished as prosocoele, thalamoccele, mesocoele, epi- 

 coele and metaccele, and instead of being uniform in diameter, like the central canal of 

 the cord, differ very much from 

 each other in configuration. The 

 same is true of the walls of these 

 various cavities, the growth in 

 thickness of which is by no 

 means uniform, so that the floor 

 of such a cavity may be very 

 thick, while its roof is extremely 

 thin. So also the five regions do 

 not ultimately retain an approxi- 

 mately equal size, but one or 

 other becomes developed in ex- 

 cess of the others, so as to over- 

 lap them. Another modifying 

 factor in determining the shape 

 of the brain is the marked bend 

 in the head of the embryo, which 

 brings the mesencephalon to oc- 

 cupy its extreme anterior end, 

 and which simultaneously short- 

 ens the floor of this region of the 

 brain. 



As we regard that part of the skull in front of the notochord as different from the 

 hinder part, so we may distinguish the prechordal part of the brain from the hinder 

 epichordal part, the limit between the two being approximately that between the floor 

 of the thalamocoele and mesocoele. 



Fig. 33.— Horizontal sections through head of young gar-pilie, .^ on a 

 lower, B on a higher plane ; chondrocranium dotted ; bram -with 

 diagonal shading ; 1 prosocoele ; 2, thalamocoele ; 3, mesocoele j 4 

 epicoele ; 5, metacoele. 



Fig. 34. — Mesal sagittal section of gar-pike's brain. The dotted lines, 1 — 8, indicate the positions of 

 the frontal sections in Fig. 35 ; r, rhinencephalon ; jf, thin septum of prosocoele ; o, ojjtic nerve ; 2, 

 thalamocoele ; e, epiphysis : h, hypophysis ; 3, mesocoele (the curved dotted line indicates the 

 limits of its floor) ; 4, epicoele ; 5, metaccele. 



Instead of a mere diagram to explain the relationship of the various regions of 

 the brain to each other, Figs. 33 to 35, which represent sections in different directions 

 through a comparatively primitive brain — that of the gar-pike — will serve as an ad- 



