DEVELOPMENT OF THE NERVOUS SYSTEM. 63 



a part of the cells become unipolar and in higher vertebrates 

 most of them do so. This change is brought about by a sort of 

 bending of the cell body by which the fibers at the two ends are 

 brought together at one side (Fig. 35). Then by the growth of 

 that part of the cell to which the fibers are attached both fibers 

 come to be borne on a single process. 



The development of the motor nerves takes place somewhat 

 later than that of the sensory nerves. In the ventral part of the 

 spinal cord and brain, where motor nerves are to be formed, 

 certain large neuroblasts send their neurites out on the ventral 

 surface. At this period numerous cells wander out of the cord or 

 brain, following the neurites in their growth, migrate and multiply 

 and eventually form the sheath of Schwann (cf. p. 51 above). 

 The neurites grow until they reach the muscles which they are to 

 innervate, when they branch or expand on the surface of the 

 muscle fibers in the form of special motor end organs. These 

 ventral motor nerves innervate only those muscles which are 

 derived from the mesodermic somites. It has recently been 

 shown that in amphibia fibers of a single nerv^e are distributed 

 to muscle fibers of two adjacent muscle segments. This fact 

 is wholly inconsistent with the view held by some anatomists 

 that from the earliest embryonic stages processes of nerve cells 

 are in connection with their muscles by means of strands of proto- 

 plasm which become elongated to form motor nerve fibers. 



The nerve fibers which innervate the muscles derived from the 

 lateral mesoderm are developed in the same way from neuro- 

 blasts which lie in the lateral parts of the central gray. In lower 

 vertebrates these fibers pass out of the cord or brain together with 

 or close beneath the dorsal sensory nerve roots and pass through 

 the sensory ganglia. In higher vertebrates a part of these fibers 

 pass out with the ventral roots and in the cat, dog and monkey 

 all of them are said to have this course. 



Morphology of the head. — In order to understand the 

 brain and cranial nerves it is necessary to take into account the 

 morphology of the organs of the head. For this reason a sum- 

 mary of the more important conclusions of comparative embryology 

 and anatomy will be given here. 



