i 6 BRAIN 



(habenular). Between these two tracts of fibres are the epiphysial 

 outgrowths. At the anterior limit the thin non-nervous portions 

 of the roof form a choroid plexus, sinking down behind the 

 paraphysis, and projecting into the large 3rd ventricle as a velum 

 transversum (permanent in the lower and transient in the higher 

 forms). At the sides arise the optic thalami ; above the ganglia 

 habenulae. Below is the large infundibular downgrowth, with the 

 optic chiasma and nerves immediately in front. The foremost 

 division of the brain is that which undergoes the most conspicuous 

 change in phylogenetic differentiation. Its hind limit above is 

 marked by the commissui'a habenularis, and below by the recessus 



Fig. 9. 



Diagram of the divisions of the brain. (After von Kuppfer, from Hertwig's Handbuch.) a, a, 

 d, '', /, /, limits between the regions ; Ml, myelencephalon ; Mt, metencephalon ; M, mesen- 

 cephalon : D, diencephalon ; T, telencephalon ; c.c, comtnissnra cerebellaris ; c.h, commissura 

 habenularis ; p.n, processus neuroporicus ; p.r, plica rhombo-mesencephalica : p. u, plica ence- 

 phali ventrali. Other letters as in Fig. 10. 



opticus, marking the region from which develop the paired optic 

 vesicles at a very early stage. In the middle line in front the wall 

 forms the lamina terminalis, across which pass the anterior dorsal 

 and ventral commissures. The thin roof projects upwards as the 

 paraphysis (p. 25). This region between the recessus opticus and 

 the lamina terminalis is the telencephalon. But the bulk of the 

 secondary fore-brain becomes differentiated into large paired out- 

 growths, into which extends the 3rd ventricle. These are the 

 cerebrum (prosencephalon) and the olfactory lobe (rhinencephalon). 

 In the higher forms the thick-walled hollow outgrowths become 

 very distinctly paired cerebral hemispheres passing far in front of 

 the lamina terminalis. The corpus striatum is a thickening on 

 their outer ventral wall. The communication of their cavities on 

 either side with the median 3rd ventricle (prosocoele) narrows into 



