THE NEURAL TUBE AND ITS DERIVATIVES 25 



rhombencephalon (Fig. 7). The more rostral vesicle becomes subdivided by a 

 constriction into the telencephalon and diencephalon (Fig. 7, B, C). The rhom- 

 bencephalon is less sharply subdivided into a rostral part, which includes the 

 cerebellum, and is known as the metencephalon, and a more caudal portion, the 

 myelencephalon. The optic nerves and retinae, not illustrated in the figure, 

 develop as paired evaginations from the prosencephalon. 



The Cerebral Hemispheres. The telencephalon includes a thickened portion 

 of the ventrolateral wall loosely designated as the corpus striatum or, since there 



Fig. 7. Diagrams illustrating the development of the vertebrate brain: A, First stage, side 

 view, the cavity indicated by dotted line; B, second stage; C, third stage, side view of a brain with- 

 out cerebral hemispheres; D, the same in sagittal section; E, fourth stage, side view of a brain with 

 cerebral hemispheres; F, the same in sagittal section; G, dorsal view of the same with the cavities 

 exposed on the right side. Rhin., rhinocoele; Lot. Vent., lateral ventricle; Int. For., interventricu- 

 lar foramen; Vent. Ill, third ventricle; Vent. IV, fourth ventricle. /, Prosencephalon; / a, Telen- 

 cephalon; I a-r, Rhinencephalon ; I a-p, Pallium; / a-lt, Lamina terminalis; / a-ch, Cerebral 

 hemisphere; i a-cs, Corpus striatum; i b, Diencephalon; / b-t, Thalamus. 2, Mesencephalon ; 2c, 

 Optic lobes; 2 d, Crura cerebri. j, Rhombencephalon; j a, Metencephalon; 3 a-c, Cerebellum; 

 3 b, Myelencephalon. 



is one of these on either side, the corpora striata (Fig. 7, D). Another part of 

 the wall is relatively thin and is known as the pallium, while the part directly 

 associated with the olfactory nerve belongs to the rhinencephalon. The most 

 important factor in the evolution of the vertebrate brain is the progressive evag- 

 ination of the lateral walls of the telencephalon to form paired masses, the 

 cerebral hemispheres. In primitive forms like the cyclostomes only a part of the 

 rhinencephalon has been evaginated, and in them the hemisphere consists only 

 of an olfactory bulb and olfactory lobe. This stage of development is roughly 



