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. 5 1 ). 



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- 



