THE NERVOUS SYSTEM. 485 



flexure. The pallium has increased in size and now forms a considerable 

 prominence on the brain tube. Its boundaries are also much more clearly 

 marked off (see Fig. 471). On the inner side of the tube, the area below 

 the bulging of the pallium is the corpus striatum. Externally, just below the 

 bulging, we have the region where the olfactory lobes are differentiated. The 

 proximal part of the optic evagination has become longer and narrower. The 

 ventral expansion of the diencephalon is the hypothalamus, the portion of the 

 diencephalon dorsal to the latter being the thalamus. Two slight protrusions 

 of the ventral wall of the hypothalamus have appeared; the caudal one is the 

 mammillary region, the anterior one the infundibulum. The cavity of the 

 diencephalon (third ventricle] is connected by the mid-brain cavity (iter or 

 aquaductus Sylvii) with the rhombic brain cavity or fourth ventricle. 



HISTOGENESIS OF THE NERVOUS SYSTEM. 



The neural plate is at first a simple columnar epithelium. The various 

 processes by which this is converted into the fully formed nervous system are : 

 (i) cell proliferation; (2) cell migration; (3) cell differentiation. These proc- 

 esses are not entirely successive in point of time, but overlap each other. Cell 

 division is present from the first, increases to a certain period in development 

 and then practically ceases; cell migration is partly a necessary concomitant and 

 resultant of cell division, and cell differentiation is in part due to the growth of 

 the cytoplasm and is in part a result of environmental differences produced by 

 these processes. In development the following stages may be distinguished : 



(i) Stage of indifferent epithelium; (2) appearance of nerve elements 

 (neurones) and resulting differentiation into supporting and nerve elements; 

 (3) growth of neurones and resulting differentiation and development of (a) 

 peripheral neurones, (b) lower intermediate neurones, (c) neurones of higher 

 centers and neurone groups in connection with them. These stages do not 

 occur simultaneously throughout the whole neural tub, some parts being more 

 backward in development than others. This fact has already been alluded to 

 (p. 480). In general the spinal cord and epichordal segmental brain are most 

 advanced in development. Furthermore, the ventral part of the brain tube 

 precedes the dorsal. The most backward part of the whole neural tube is the 

 pallium. 



The various phases of /0;'w-differentiation of the neurone are (i) the 

 development of the axone and, later, of its branches; (2) the growth of the 

 dendrites; (3) the formation of accessory coverings or sheaths, the neurilemma 

 and the myelin (medullary) sheath. The principal internal differentiations 

 are (i) the appearance of the neurofibrils; (2) the chromophilic bodies of 

 Nissl; (3) pigment. These latter may all be regarded as products of the 

 nucleus and undifferentiated cytoplasm of the nerve-cell. 



