NERVOUS SYSTEMS OF BACKBONED ANIMALS 21 
Amphibians, Reptiles, Birds, and Mammals look so extremely 
unlike that comparison seems hopeless, but such an idea is soon 
dispelled by a study of de 
velopment, which is the key 
to the whole matter. What ” CON a 
m —_ 
takes place is broadly as fol- 
Fo OM. HL Sp.C. 
lows (fig. 1025). The front FE. M. H. 
end of the nerve-tube grows Pn. | 
Olf. = C.H. i T,B OL cb 
rapidly, and divides into three os 
successive swellings, which TIC SS lan PN a“ 
we may term the Fore-, Mid-, cr 
and Hind-~- Brains. These / ee ersallgmmoaaony) 
oe ‘ YYyyf Z UE ALLIED LT Ta 
three original swellings are 9, 7. nn 
Py, 
converted into the central Fig. 1025.—Development of Vertebrate Brain, as seen in 
part or axis of the adult longitudinal section, diagrammatic 
brain, the front part of which a, Brain and spinal cord at early stage; B, brain at later 
7 : . : stage enclosed in brain-case, the floor of which is shaded with 
is called the ’Twixt-Brain,and oblique lines; 7, 7, #., S#.C., fore-, mid-, and hind-brains, 
4 and spinal cord; 7. B., ’twixt-brain; C.H., cerebral hemispheres; 
the hind par t the Medulla OUf,, olfactory lobe projecting into nasal capsule; Pz. and Py., 
: pineal and pituitary bodies; O.Z., optic lobes; J7.0., medulla 
Oblongata or Spinal Bulb oblongata; Cé., cerebellum; WV. W., notochord; 11, optic nerve. 
(continuous behind with the 
Spinal Cord), while the roof of the middle section is thickened 
Fig. 1026.—Brains of Trout (a), Frog (B), and Dog (c), seen from above, and drawn same length 
OUf,, Olfactory lobes; C.H., cerebral hemispheres (CZ. is a cleft between them in B); Fs; “twixt-brain ; Px., pineal 
body; O.Z., optic lobes; C2., cerebellum; J7.0., medulla oblongata; V., V., WV., cranial nerves; 7., 7., spinal nerves. 
into a pair of swellings known as Optic Lobes, each of which, 
in Mammals only, is divided into two smaller projections by a 
