342 



DEVELOPMENT OF THE HUMAN BRAIN 



walls of the Hemispheres and also of the Optic Lobes 

 have acquired a much greater thickness and the principal 

 * commissures ' have in great part assumed their typical 

 condition. This is the case more especially with the 

 Corpus Callosum and the Fornix, between which the 

 ' fifth ventricle ' has begun to appear. The two halves 

 of the Middle Commissure have also grown together. 



During the same period the 

 Cerebellum has undergone 

 important changes. From the 

 end of the fourth month the 

 development of its * lateral 

 lobes ' takes place at a slower 

 rate, and the previously absent 

 ' median lobe ' not only begins 

 to appear but also becomes 

 marked on the surface by 

 three or four transverse folds. 

 The ' lateral lobes ' are still 

 perfectly smooth though by 



fli" end of the sixth month 



after R. Wagner.) Letters of reference they also have acquired nu- 

 as in last figure. /> 



; merous transverse fissures. 



The pons Varolii, as already intimated, undergoes a 

 development correlative with that of the lateral lobes of 

 the Cerebellum. 



In the remaining important section of intrauterine life, 

 from the 6th to the end of the 9th month, the develop- 

 mental -changes in the Cerebrum are much more marked 

 than they are in the Cerebellum. The walls of the Cerebral 

 Hemispheres become thicker, and there is a proportionate 

 diminution in the capacity of the * lateral ventricles,' the 

 three ' horns ' of which now become quite distinct. The 

 Corpus Callosum assumes a more horizontal direction, 



