130 THE STRUCTURE OF THE FOWL 



before the end of the first day of incubation in 

 the form of the neural groove bounded on each 

 side by a medullary fold (Fig. 58). By approxi- 

 mation and union of the folds the groove is 

 converted into a tube. Three dilatations or 

 vesicles are produced in that part of the tube 

 which will ultimately become the brain ; the 

 rest of the tube — from which the spinal cord 

 develops — remaining of fairly uniform calibre 

 throughout. The three primary dilatations are 

 known as the fore -brain, the mid -brain, and 

 the hind-brain vesicles (Fig. 70), and from each 

 certain definite parts of the adult brain are 

 formed. The wall of the hind-brain vesicle 

 is early marked by transverse constrictions 

 demarcating segments known as neuromeres, 

 six in number. Three comparable neuromeres 

 have been described as occurring in the fore- 

 brain, and two in the mid-brain : making 

 eleven in all. The original lumen of the 

 neural tube is represented in the adult by the 

 central canal of the spinal cord and the 

 ventricles of the brain. 



Even before the union of the medullary 

 folds is effected, lateral expansions of the wall 

 of the anterior end of the neural tube mark 

 the first traces of the optic vesicles from which 



