DEVELOPMENT OF THE BEAIN AND GANGLIA. 



49 



Physiologically these things afEord an especial interest, and it is to be 

 hoped that they will some time afford also an especial interest psycholog- 

 ically. It is certainly to be regretted that these subjects are here still quite 

 insufficiently recognized. 



Prom the foregoing chapters we know that the entire fundament of the 

 nervous system is furnished by the outer germ-layer, — epiblast, — that this 

 fundament forms a plate which; soon sinks in to form a groove. Very early 

 the medullary groove becomes closed, forming a medullary canal. But even 

 before this closure is complete one may recognize in all vertebrates, in the 

 region where the brain is developing, three vesicular expansions: the 

 (primitive) f orebrain, midbrain, and hindbrain. 



The wall which closes the forebrain anteriorly is called the embryonic 

 terminal lamina; the closure of the original medullary plate results in a 

 seam, or raphe. 



Fig. 19. — Longitudinal section tlirough tlie brain of a newborn Idtten. The 

 Thalamus {Zwischenhirn ) and Corpora Quadrigemina {Mittlehirn) covered by the 

 Cerebrum CVorherhirn) . 



The section of the brain of a larval sturgeon (see Pig. 18) contains, as 

 the description will show, the most varied fundaments for the further de- 

 velopment of the separate brain-segments. iTot all come to maturity; many 

 remain in the stage here shown; but among the higher vertebrates the 

 separate smaller segments of the brain-tube become metamorphosed into im- 

 portant structures, whose development may be very different for the differ- 

 ent classes. 



Pirst note the small epithelial plate at the dorsal end of the lamina ter- 

 minalis. 



In most vertebrates there arises out of that part of the forebrain which 

 lies dorsally and laterally from the lamina terminalis a new and important 

 structure, the Prosencephalon: a large vesicle located anteriorly and dor- 

 sally, which is soon divided into right and left hemispheres by a longitudinal 



