THE BRAIN. 4OI 



irregular process,^ dendrite, is thrown out from the peripheral end of the 

 cell; later it becomes smooth and fibrillar in character. This bipolar condi- 

 tion persists in very low forms, such as the amphyoxus and cyclostomes. 

 4. In bony fishes some of the neurones become unipolar. The greater num- 

 ber become unipolar in higher vertebrates (/. B. Johnston's Nervous System 

 of Vertebrates). 



(2) The spinal portion of the neural crest forms the thirty-one 

 pairs of spinal ganglia situated on the posterior nerve roots; 

 and, also, the vertebral, prevertebral and terminal ganglia of the 

 sympathetic system. The sympathetic ganglia wander widely. 

 In them the epiblastic cells develop into multipolar neurones, 

 the nonmedullated processes of which constitute the larger num- 

 ber of gray fibers in the sympathetic system. The cells of the 

 spinal ganglia form unipolar neurones, like those of the vagus, 

 glossopharyngeal, geniculate and semilunar ganglia. Like them, 

 also, the single processes divide, T-like, the peripheral arms 

 of the T-branches forming the sensory part of each spinal nerve 

 and the central arms (the axones) the posterior roots of those 

 nerves. The latter enter the cord at the posterior lateral sulcus 

 and, before and after dividing T-like into an ascending and a 

 descending branch, give off collaterals. The descending -fibers, 

 long and short, arborize and end in the gray matter of the posterior 

 columna, the center of the crescent and the anterior columna, the 

 long fibers forming the postero-medial and postero-lateral descend- 

 ing radicular tracts; the ascending axones and collaterals termin- 

 ate in the gray substance of the spinal cord and in the nuclei 

 of its posterior columns, namely, the nucleus funiculi gracilis 

 and nucleus funiculi cuneati of the medulla. The long ascend- 

 ing fibers form the postero-medial tract (Goll's) and, the postero- 

 lateral tract (Burdach's); and, the shorter ones, those reaching 

 but a few segments, make up the marginal tract of Lissauer. 

 The greater number of posterior root-fibers terminate in the gray 

 substance at or near their level of entrance into the cord. 



THE BRAIN. 



The primary brain vesicles grow rapidly. By the end of the 



fourth week a constriction is visible in the anterior primary vesicle 

 26 



