PEIMITIVE FOEM OF THE BRAIN. 



(51 



any other parts ; so that the head of the embryo consists at first of 

 no more than the wider part of the medullary tube and the simple 

 enclosing wall. 



Fig. 55L 



Fig. 551. — Four Views of the Brain of an Embrto-kitten in the Stage of First 

 Division into the Five Cerebral Eudiments, magnified Three Diameters (from 

 Reicliert). 



A, from above ; B, from the side ; C, vertical section sliowiug the interior ; D, from 

 helow. 



1, Cerebral hemisphere, prosencephalon ; 2, thalamencephalon ; 3, mesencephalon, 

 still single ; 4, cerebellum, epencephalon ; 5, mj-elencephalon, medulla oblongata ; o, 

 optic nerves ; V, fifth ^pair ; "VIII, eighth pair or glossopharyngeal and pneumogastric ; 

 i, infundibulum ; v, v', general ventricular cavity, opening at v, into the lateral ventricle 

 by the foramen of Monro. 



In the base of this wall, it will be remembered that the notochord 

 extends forward beneath the posterior and middle of the vesicles, and 

 occupies, therefore, the part of the cranium corresponding to the occi- 

 pito-sphenoidal basis, while the trabecute cranii, developed forwards 



Fig. 552. 



Fig. 552. — Vertical Sections of Embryonic 

 Brains in two Stages of Transition from 

 the Rudimentary Condition, Magnified 

 THREE Diameters (from Eeichert). 



A, Brain of the embryo pig in commencing 

 state of transition. 1, Right cerebral hemi- 

 sphere ; 2, thalamencephalon and position of 

 the pineal gland ; 3, midbrain, with a large 

 cavity ; /, foramen of Monro ; i, infundibulum ; 

 4, cereliellum ; 5, medulla oblongata. 



B, Brain of the embryo of the cat more 

 advanced, c, Cerebral hemisphere passing back- 

 wards so as to cover the other parts in succes- 

 sion ; I, olfactory bulb ; II, optic ner^'e ; th, 

 thalamus opticus ; /, foramen of Monro ; cc, cor- 

 25US callosum ; ^3, pineal gland ; i, infundibulum ; 

 cq, corpora quadrigemina, not yet divided ; 3, 

 third ventricle ; cr, crura cerebri, the aque- 

 duct of Sylvius, now reduced in width ; c', cere- 

 bellum ; 4, fourth ventricle ; jpv, Pons Varolii ; 

 mi, medulla oblongata. 



from below the anterior vesicle, are prolonged in the anterior or spheno- 

 ethmoidal part. The latter cerebral rudiment, therefore, which corre- 

 sponds to the thalami optici and third ventricle, and which may with 

 Huxley be conveniently called thaJamcncepiialon, is at first the foremost 

 part of the brain, and the region of the ijituitary fossa lying below it is 

 the foremost part of the cranial basis. The manner in which the deve- 

 lopment of the trabecule and other elements of the face modifies the 



