496 



TEXT-BOOK OF EMBRYOLOGY. 



While the structure thus formed expands enormously in a lateral direction, 

 in its subsequent development its greatest growth is in a longitudinal direction. 

 The effect of this is that the continuations of the cerebellum forward (velum 

 medullare anterius) and backward (velum medullare posterius) into the adjoining, 

 brain walls of the isthmus and medulla are comparatively fixed points and are 

 completely overlapped by the spreading cerebellum, producing an appearance 

 in sagittal section as though they were rolled in under the latter structure (comp. 

 Fig. 370, F). Another result of this longitudinal growth is the formation of fis- 

 sures running across the organ, transversely to the longitudinal brain axis. 

 First, lateral incisures separate two caudal lateral portions, the flocculi (Fig. 

 419), the median continuation of which, the nodule, is finally rolled in on 

 the under side of the cerebellum as explained above. Another transverse fissure, 

 the primary fissure, beginning in the median part and extending laterally, sepa- 



Cerebellar hemisphere 



Taenia 



Tuberculum cuneatum - 



Clava-""' 



Tuberculum cinereum (Rolando) 



- Vermis 



Eminentia teres 



Taenia 



Fasciculus gracilis (Goll) 

 Fasciculus cuneatus (Burdach) 



FIG. 419. Dorsal view of the cerebellum and medulla of a 5 months' human fetus. Kollmann. 



rates an anterior lobe from a middle lobe, the former comprising the future lin- 

 gula, centralis and culmen and their lateral extensions. The anterior portion 

 is rolled forward under the anterior part of the cerebellum. Another trans- 

 verse fissure next appears in the median part (secondary fissure} which later ex- 

 tends (peritonsillar) to the floccular incisure, and thereby completes the de- 

 marcation of a posterior lobe, including not only the flocculus and nodule, but 

 also the tonsilla and uvula, which are also rolled backward and under. The 

 result of this transverse fissuration would be the production of a cerebellum 

 resembling that of certain forms below Mammals where the cerebellum is well 

 developed (Selachians, Birds). A complicating factor, however, is the great 

 growth of certain lateral portions of the middle lobe, forming the future cere- 

 bellar hemispheres (Fig. 419), which causes also a lateral overlapping and rolling 

 inward of adjoining parts. This growth is the chief factor in the division 

 of the cerebellum into vermis and hemispheres and is correlated with the devel- 

 opment of the neopallium (p. 436 and Fig. 371). 





