Development of the Second Day 155 



The optic vesicles become considerably elon- 

 gated and are constricted at their bases into 

 stalks. Instead of projecting straight out 

 from the sides of the fore-brain, they are now 

 pressed downwards and backwards. 



The cranial nerves make their appearance 

 at the end of this period, but their develop- 

 ment will be described later on. 



Owing, probably, to the more rapid growth 

 of the dorsal wall of the medullary canal, in 

 the region of the mid-brain, the brain becomes 

 bent downwards, around the anterior end of 

 the notochord, at the end of this period, just 

 as it did in the frog (page 36) : this downward 

 bending of the brain is known as cranial flex- 

 ure (Fig. 51). 



The notochord, whose origin during the first 

 day has been described, is by this time a con- 

 spicuous cylindrical rod, lying under the medul- 

 lary canal for the greater part of its length 



(Fig. 54). 



The heart, by the end of this period, has 

 become still more markedly bent and twisted, 

 so that it is now somewhat S-shaped, with the 

 venous end rather above and behind the arterial 

 end. The venous and arterial ends have ap- 

 parently come close together, with the inter- 



