464 EMBRYOLOGY. 



11. The separate parts of the brain are derivable from the five 

 brain- vesicles ; the accompanying table (MIHALKOVICS, SCHWALBE) 

 gives a survey of the subject. 



12. In the metamorphoses of the vesicles the following processes 

 take place : (a) certain regions of the walls become more or less 

 thickened, whereas other regions undergo a diminution in thickness 

 and do not develop nervous substance (roof -plates of the third and 

 fourth ventricles) ; (6) the walls of the vesicles are infolded ; 

 (c) some of the vesicles (first and fourth) greatly exceed in their 

 growth the remaining ones (between-brain, mid-brain, after-brain, or 

 medulla oblongata). 



13. The four ventricles of the brain and the aqueductus Sylvii 

 are derived from the cavities of the vesicles. 



14. Of the five vesicles that of the mid-brain is the most conser- 

 vative and undergoes the least metamorphosis. 



15. The vesicles of the between-brain and after-brain exhibit 

 similar alterations : their upper walls or roof -plates are reduced in 

 thickness to a single layer of epithelial cells, and in conjunction 

 with the growing pia mater produce the choroid plexuses (anterior, 

 lateral, posterior choroid plexus ; anterior, posterior brain-fissure). 



16. The cerebral vesicle is divided by the development of the 

 longitudinal (interpallial) fissure and the falx cerebri into lateral 

 halves, the two vesicles of the cerebral hemispheres. 



1 7. In Man the cerebral hemispheres finally exceed in volume all 

 the remaining parts of the brain, and grow from above and from the 

 sides as cerebral mantle over the other brain-vesicles (from the second 

 to the fifth inclusive) or the brain-stalk. 



18. In the folding of the walls of the hemispheres there are to be 

 distinguished fissures and sulci. 



19. The fissures (fossa Sylvii, fissura hippocampi, fissura choroidea, 

 fissura calcarina, fissura occipitalis) are complete folds of the wall of 

 the brain, by means of which there are produced deep incisions in 

 the surface and corresponding projections into the lateral ventricles 

 (corpus striatum, cornu Ammonis, fold of the choroid plexus, calcar 

 avis). 



20. The sulci are incisions limited to the cortical portion of the 

 wall of the brain, and are deeper or shallower according to the time 

 of their formation (primary, secondary, tertiary sulci). 



21. In general the fissures appear earlier than the sulci. 



22. The olfactory nerve is not equivalent to a peripheral nerve- 

 trunk, but, like the optic vesicle and optic nerve, a special part of 



