THE NERVOUS SYSTEM. 



489 



apparently indicates the beginning of this differentiation. The significance 

 of the dorso-lateral nucleus is uncertain. It possibly sends fibers to the 

 sympathetic system. 



At about this period six transverse rhombic grooves are plainly marked in 

 the floor of the fourth ventricle, standing in relation with the nerves of this 

 region (Fig. 428). They are ordinarily regarded as neuromeric, but the above 

 relation would indicate that they have primarily a branchiomeric character 

 (Streeter). It will be noticed that each of the three main ganglionic masses 

 of this region (p. 495) corresponds to two of the grooves. (Comp. p. 465). 



The further development of the efferent neurones exhibits phases common 

 to many other nerve-cells with a large amount of cytoplasm (somatochrome 

 cells). The further development of the neuro fibrils of cell body and dendrites 



Neural crest 



Ectoderm 



Neural crest 



c 



-^'Primitive 

 segment 



FIG. 429. Three stages in the closure of the neural tube and formation of the neural crest (spinal 

 ganglion rudiment). From transverse sections of a human embryo of 2.5 mm. (13 pairs of 

 primitive segments, 14-16 days), von Lenhossek. 



is, according to some observations, at first confined to the peripheral portions, 

 leaving a clear zone in the vicinity of the nucleus. The chromophilic sub- 

 stance first appears as distinct granules about the end of the second month, 

 there being apparently a diffuse chromophilic substance present before this 

 period. The chromophilic granules also are first differentiated in the per- 

 ipheral portions of the cell. A still later differentiation is the pigment, which 

 probably does not appear till after birth. This increases greatly in amount 

 in later years and is then an indication of senility of the nerve-cell. 



Afferent Peripheral and Sympathetic Neurones. It has already been 

 mentioned (p. 451) that in the closure of the neural tube certain cells forming 

 an intermediate band between the borders of the neural plate and the non- 

 neural ectoderm are brought together by the fusion of the lips of the plate 



