50 NERVOUS SYSTEM OF VERTEBRATES. 



shared in the segmentation of the neural tube. The cells of the 

 crest multiply by mitosis and in the trimk region each segment 

 moves latero-ventrad at the sides of the spinal cord, between it 

 and the adjacent ectoderm and mesoderm. At the same time 

 the segments separate more and more from one another until 

 there are formed a series of flaps attached to the dorso-lateral 

 surface of the cord and hanging down on its lateral surface opposite 

 the interval between each two mesodermic somites. Each of these 

 segments of the neural crest may now be spoken of as the anlage 

 of a spinal ganglion. Each anlage moves further ventrad and 

 may wholly lose its connection with the cord (see Figs. 15, 16). 

 The cells are now elongated, spindle-shaped and it is soon noticed 

 that each cell elongates further as if each end were being drawn 

 out into a slender thread. In fact a process analogous to a pseu- 

 dopodium grows out from each end of the cell. The process 

 from the upper end of the cell, or central process, appears first 

 and is from the first distinctly more slender than the opposite,, 

 or pheripheral process. The central process is therefore to be 

 considered as the neurite, the peripheral process as the dendrite. 

 Figure 26 is from part of a transverse section of an embryo of 

 Amhly stoma punctatum at the level of the glosso-pharyngeus 

 ganghon. The central processes of the ganglion cells are growing 

 upward, the peripheral processes are not yet formed. Figure 

 27 shows a few ganglion cells of the trigeminus nerve. The per- 

 ipheral processes of the cells are seen but the central processes 

 bend backward so that they do not fall in the same section. 



The central processes soon reach the dorso-lateral surface of 

 the spinal cord and grow into the cord where they form a bundle 

 of afferent or sensory nerve fibers. The peripheral fiber grows 

 also and with its fellows forms a sensory nerve or the sensory 

 portion of a peripheral nerve trunk. The peripheral process 

 sometimes divides, its branches entering two nerve rami. This 

 is analogous to the branching of the dendrites of central nerve 

 cells. The fibers continue their growth, following the path of 

 least resistance between the muscles and other organs, until they 

 reach the area of their ultimate distribution, for example in the 

 skin, where thev divide into terminal branches. The cells 



