334 



DEVELOPMENT OF THE HUMAN BRAIN 



are developed other portions of the Cerebral Peduncles and also 

 the Thalami which rest upon and grow as ganglionic thickenings 

 from the latter structures. Its diminished cavity persists as the 

 future Third Ventricle (a). Its roof becomes gradually thinned 



FIG. 122. Diagrams illustrating the Progressive Changes that take place during 

 the Early Stages of the Development of the Brain. (Mivart.) 



1. Early condition of Brain when it consists of three hollow vesicles (a 6 c), the 

 cavity of which is continuous with the wide cavity (d) of the primitive Spinal 

 Cord (ra). 



2. Here the first vesicle or Fore-brain has developed the Pineal Body (pi) above, 

 and the Pituitary Body ( p f) below. The wall at the anterior end of the first vesicle 

 is the future ' lamina terminalis ' (t). 



3. This figure shows the Cerebrum (c r) budding from the first vesicle, its anterio" 

 part (o) being prolonged as the Olfactory Lobe ; the cavi< 7 of the Cerebrum (the 

 incipient 'lateral ventricle') communicating with that c. c che Olfactory Lobe in 

 frout, and with that of the first Cerebral Vesicle behind (this latter cavity persisting 

 as the future ' third ventricle '). The latter communication takes place through the 

 'foramen of Monro.' The walls of the three primitive vesicles are becoming of 

 unequal thickness, and the cavity (6) of the middle vesicle is becoming reduced in 

 relative size. 



4. The Cerebrum is here enlarged, and the inequality in thickness of the wall of 

 the primitive vesicles is increased. The greater development of the Cerebellum (c b), 

 the Pons (p), and the Corpora Quadrigemina (q), show this distinctly. 



5. This figure shows the Cerebrum still more enlarged, and containing a tri-radiate 

 cavity (I, 1, 2, 3). The part destined to form the Fornix (/), which in No. 4 was 

 above, has now come to look slightly downwards, and prolongations from it begin to 

 extend towards the ' corpora albicantia' (MI a). v t corresponds with the situation of 

 tho 'velum interpositum.' 



