THE ORGANS OF THE OUTER GERM-LAYER. 425 



vesicle of the cerebellum and that of the after-brain, and has the 

 form of a ridge which projects veiitrally for a considerable distance, 

 where subsequently the transverse fibres of the pons Yarolii are 

 established. 



The extent of these curvatures is very different in the various 

 classes of Vertebrates. Thus the cephalic flexure is only s'ightly 

 emphasised in the lower Vertebrates (Cyclostomes, Fishes, Amphibia) ; 

 it is, on the contrary, much greater in Reptiles, Birds, and Mammals ; 

 but in Man especially, whose brain is the most voluminous, all of the 

 flexures are developed to a very high degree. 



The five brain-vesicles furnish the foundation for a natural sub- 

 division of the brain, whose various chief divisions can be referred 

 back to them. As the study of the further development teaches, 

 there are formed from 



the after-brain vesicle ^^ m k 



the medulla oblongata, 

 from the vesicle of the 

 cerebellum the vermi- 

 form process with the 

 hemispheres of the cere- 

 bellum and the pons 



^ . Fig. 239. Brain of a Rabbit embryo 16 mm. long, viewed 



Varolll, trom the mid- from the left side. The outer wall of the left cerebrum 



brain vesicle the crura is removed. After MIHALKOVICS. 



sn, Optic nerve ; ML, foramen of MONRO ; agf, fold of the 



Cerebri and Corpora choroid plexus ; amf, fold of the oornu Ammonia ; 



quadrigeinina, from the ** ^tween-brain : mh, mid-brain (cephalic or parietal 



1 I t flexure) ; kh, cerebellum ; Dp, roof -plate of the fourth 



between - brain Vesicle ventricle ; 66, pontal flexure ; mo, medulla oblongata. 



the bet wee n-b rain 



[thalamencephalon] with the inf undibulum, the pineal gland, and the 

 optic thalami, and finally from the vesicle of the cerebrum the 

 cerebral hemispheres. 



Tn this metamorphosis the cavities of the primitive cerebral tube 

 become the so-called ventricles of the brain : from the cavities of the 

 fourth and fifth vesicles is derived the fourth ventricle or fossa 

 rhomboidalis ; from the cavity of the mid-brain vesicle, the aque- 

 duct of SYLVIUS ; from the between-brain, the third ventricle ; and 

 finally from the cavities of the hemispheres, the two lateral ventricles, 

 which are also designated as the first and second ventricles. 



A brief sketch will suffice to show in what manner the most 

 important parts of the brain develop out of the five vesicular 

 fundaments, and that at the same time histological and morphological 

 differentiations are most intimately associated. 



