548 



COMPARATIVE ANATOMY 



roof of the mouth. The pars nervosa of the pituitary is proliferated from 

 its ventral extremity during the third month. 



Mesencephalon. The inesencephalon undergoes relatively little 

 change during ontogenesis. As a result of thickening the lateral walls, 

 the central cavity is reduced to a slender aqueduct which connects third 



metacoele 

 ( ventricle tV) 

 thin roof of myelcncephalon 



myelocoele 

 / ventricle IV ) 



posterior commissure 

 esodiencephalic fold 



(ventricle III ; 



foramen of Monro 



(Sylvian aqueduct) 



Fig. 452. — Diagrams to show the topography of the brain of a four-day chick. 

 A, plan of sagittal section. The arbitrary boundaries between the various brain 

 vesicles (according to Kupffer) are indicated by broken lines. B, dextral view of a 

 brain which has been dissected free. C, schematic frontal section plan of brain. The 

 flexures of the brain are supposed to have been straightened before the section was cut. 

 (From Patten's "Embryology of the Chick.") 



and fourth ventricles. From the dorsal portion of the midbrain arise 

 the corpora quadrigemina, which contain optic and auditory reflex 

 centers. From the dorsal plate of the mesencephalon develop also the 

 red nuclei, which receive fibers from the cerebellum and send fibers by 

 the rubrospinal tract into the spinal cord. 



The motor nucleus of the oculomotor nerve develops in the basal 

 plate. Posterior to it at the same level is the nucleus of the trochlear 

 nerve which belongs originally, in elasmobranchs, with the metencephalon. 



