36 Vertebrate Embryology 



from the epidermal layer of the original ecto- 

 blast. 



The changes that convert the anterior end 

 of the neural tube into the complicated struc- 

 tures of the adult brain are much more exten- 

 sive, and must be described in more detail, 

 although the limits of this work will not per- 

 mit a full description. 



The first indication of a separation of the 

 brain from the rest of the neural tube is seen 

 at about the time of the appearance of what is 

 known as cranial flexure. As the name would 

 indicate, cranial flexure is the bending of the 

 brain, at about its middle region, so that the 

 anterior end is pushed down, and comes to 

 lie almost at right angles with the posterior 

 part (Fig. 12). 



This cranial flexure takes place around the 

 anterior end of the notochord, and persists 

 throughout life, its apparent rectification being 

 due to inequalities in the growth of the differ- 

 ent parts of the brain. At this time the cen- 

 tral nervous system has somewhat the shape 

 of a retort, the bulb of the retort corresponding 

 to the brain, and the neck of the retort cor- 

 responding to the spinal cord. 



Two transverse folds appear very early in 



