CHAPTER IX. 



The Development of the Organs in the Head. 



Tlie Development of the Brain, 



General History. In stage Q the braia presents a very 

 simple constitution (PI. VI. fig. G), and is in fact 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, wliich is formed not so much 

 through a change in the character and arrangements of the 

 cells composing the roof, as by a divarication of the two 

 sides of the hind-brain, and the simultaneous introduction of a 

 fresh structure in the form of a thin sheet of cells connecting 

 dorsally the diverging lateral halves of this part of the brain. 

 By stage I, the hind-brain in Pristiurus also acquires an 



