CHAPTER IX. 



THE DEVELOPMENT OF THE ORGANS IN THE HEAD. 



The Development of the Brain. 



General History. In stage G the brain presents a very simple 

 constitution (PL 8, fig. G), and is in tact little more than a dilated 

 termination to the cerebro-spinal axis. Its length is nearly one- 

 third that of the whole body, being proportionately very much 

 greater than in the adult. 



It is divided by very slight constrictions into three lobes, 

 the posterior of which is considerably the largest. These are 

 known as the fore-brain, the mid-brain, and the hind-brain. 

 The anterior part of the brain is bent slightly downwards about 

 an axis passing through the mid-brain. The walls of the brain, 

 composed of several rows of elongated columnar cells, have a 

 fairly uniform thickness, and even the roof of the hind-brain 

 is as thick as any other part. Towards the end of stage G 

 the section of the hind-brain becomes somewhat triangular with 

 the apex of the triangle directed downwards. 



In Pristiurus during stage H no very important changes take 

 place in the constitution of the brain. In Scyllium, however, 

 indications appear in the hind-brain of its future division into a 

 cerebellum and medulla oblongata. The cavity of the anterior 

 part dilates and becomes rounded, while that of the posterior 

 part assumes in section an hour-glass shape, owing to an increase 

 in the thickness of the lateral parts of the walls. At the same 

 time the place of the original thick roof is taken by a very thin 

 layer, which is formed not so much through a change in the 

 character and arrangements of the cells composing the roof, as 



