V.] THE CRANIAL FLEXURE. 101 



unpaired commencement of two vesicles which develop 

 into the cerebral hemispheres ; but up to the end of the 

 day it is still very small and inconspicuous. 



Early on the second day the commencements of 

 several of the cranial nerves make their appearance 

 as outgrowths of the (Fig. 30, vg) roof of the mid and 

 hind brains, but their development, together with that 

 of the spinal nerves, will be dealt with in the next 

 chapter. 



The notochord. The notochord, whose origin 

 was described in the account of the first day, is during 

 the whole of the second day a very conspicuous object. 

 It is seen as a transparent rod, somewhat elliptical in 

 section (Fig. 34, ch), lying immediately underneath 

 the medullary canal for the greater part of its length, 

 and reaching forward in front as far as below the 

 hind border of the first cerebral vesicle. 



Cranial flexure. Round the anterior termination 

 of the notochord, the medullary canal, which up to the 

 present time has remained perfectly straight, towards 

 the end of the day begins to curve. The front portion 

 of the canal, i.e. the fore-brain with its optic and cere- 

 bral vesicles, becomes slightly bent downwards, so as to 

 form a rounded obtuse angle with the rest of the 

 embryo. This is the commencement of the so-called 

 cranial flexure and is, mechanically speaking, a con- 

 sequence of the more rapid growth of the dorsal wall of 

 the anterior part of the brain as compared with that of 

 the ventral. 



Auditory vesicle. Lastly, as far as the head is 

 concerned, the epiblastic plates forming the rudiments of 

 the auditory vesicles become converted into deep pits 



