THE NERVOUS SYSTEM. 



541 



as common to the vertebrate brain ("cushion" of the epiphysis, velum trans- 

 versum, paraphysis?) (p. 461 and Fig. 402). 



From the above it is evident that at the close of the fifth week the rudiments 

 of the various parts of the diencephalon are already well marked. These 

 rudiments are principally indicated by foldings of the walls, there being no very 

 strongly marked differences of thickness except the early differentiation between 

 the median and lateral plates. From this time on, both general and local 



Lamina terminalis 



Cavity of ant. olfact. lobe 



Anterior arcuate fissure 



Cavity cf post, olfact. lobe 



Chorioid fold 



Hippocampal fissure 



Lateral geniculate body 



Pineal region 



Ant. corp. quad. (ant. colliculus) 

 (extending forward 

 into ant. brachium) 



Angulus prathalamicus 



(a) (b) 

 (c) 



Corpus striatum 



Roof plate of diencephalon 



Fig. 464. — Dorsal view of a model of the brain of a 13.6 mm. human embryo (beginning of 6th 

 week). The' dorsal part of the pallium on each side has been removed. Compare with 

 Figs. 465 and 466. His. 



thickenings of the lateral walls occur. This indicates a rapid proliferation 

 of the cells, especially a differentiation of the nerve cells and consequent forma- 

 tion of masses of gray and white matter. Another factor affecting the dien- 

 cephalon is the subsequent growth backward over it of the cerebral 

 hemispheres. 



During the second month, the lateral walls become thickened, forming 

 a prominence on the inner surface of each side. This reduces much of the 

 cavity of the third ventricle to a cleft and in the third or fourth month a fusion of 



