THE GENERAL FUNCTION OF THE CEREBRUM 



663 



the neural tube. It is formed by an invagination of the epiblast. 1 

 Its cavity possesses a somewhat larger caliber in front than in the 

 region of the spinal cord, and becomes subdivided into three vesicles 

 by two constrictions. These are designated respectively as the fore- 

 brain, midbrain and hindbrain. To begin with, the walls of this tube 

 are thin, being composed solely of epithelial cells. The nervous 

 elements develop a little later and show a differentiation into neuro- 

 blasts and spongioblasts, the former eventually giving rise to nerve- 

 cells and the latter to the supporting tissue or neuroglia. In several 

 places, however, the original epithelium remains undifferentiated and 



Corpus itriatum 



Masenctphalon 



-Optic oesicle 



Futurt confine t; ? 

 flcx.urt b = 



Rhoinbentephalaa 



FIG. 333. AN ENLARGED MODEL OF THE BRAIN OF A HUMAN EMBRYO 3.2 MM. LONG 

 (ABOUT Two WEEKS OLD). THE OUTER SURFACE is SHOWN AT THE LEFT, AND ON THE 

 RIGHT THE INNER SURFACE AFTER DIVISION OF THE MODEL IN THE MEDIAN PLANE. THE 

 Anterior Neuropore MARKS A POINT WHERE THE NEURAL TUBE is STILL OPEN TO THE 

 SURFACE OF THE BODY. THE Pallium is THE REGION FROM WHICH THE CEREBRAL CORTEX 

 WILL DEVELOP. THE Optic recess MARKS THE PORTION OF THE LATERAL WALL OF THE 

 Diencephalon FROM WHICH.THE HOLLOW Optic vesicle HAS EVAGINATED. (After His, from 

 Prentiss' Embryology.) 



finally gives rise to" a layer of similar cells, known as the ependyma. 

 This relationship is shown best in the hindbrain, where the posterior 

 wall of the neural canal fails to develop nervous elements and reaches 

 maturity merely as a layer of epithelial cells covering an expanse of 

 the tube. This is the fourth ventricle. In other places, again, the 

 nervous elements grow very rapidly and lead to the formation of more 

 or less circumscribed structures. The cerebellum, for example, is 

 developed by an offshoot from the dorsal wall of the tube, while the 

 pons and medulla are formed by a more even outgrowth round the entire 

 central canal. The details of the development of the brain lie, of 

 course, outside the scope of this book and must be obtained from 

 works of more specialized character. 



1 Keibel and Mall, Manual of Human Embryology, Philadelphia, 1912. 



