856 



THE NERVE SYSTEM 



spheres with their great ganglia, growing with great rapidity and exceeding in 

 this respect all other parts of the brain. The growth of these secondary fore- 

 brain vesicles is principally in the distal parts, and in this manner each forms 

 a great pouch whose interior communicates with the primary neural cavity through 

 a small opening, the foramen of Monro (porta). It must be remembered that in 



Midbrain.-- P 

 Hindbrain j 



Auditory vesicle. - 



; 

 Neural >-'nl<n . 



Remains of primitive 

 streak. 



FIG. 623. Chick embryo of thirty- 



Head fold of amnion. 



Forelraiii. 

 - - Optic vesicle. 



Heart. 



\---Omplialo-mesenteric vein. 



Protovertebrse or 



|._.l mesoblastic somites. 



.StL&.M-- -Sinus rhomboidalis. 



riewed from the dorsal aspect. X 30. 



(From Duval's Atlas d'Ernbryologie.) 



these initial stages the vesicles are all extremely thin-walled, but later the walls 

 thicken or hypertrophy to a marked degree, so that the neural cavity becomes 

 relatively small. 



(c) Meanwhile (in the fourth week) the most cephalic portion of the fore-brain 

 also becomes differentiated. As the enlarging vesicles of the cerebral hemisphere 

 crowd upon the median, slower-growing portion, there is observed, on either 



