388 



CHORDATE ANATOMY 



neurilemma cells which enclose the neurites while the myehn sheaths 

 are formed. Most of these disappear in the adult cord. Some of the 

 ependymal cells persist as the epithelial lining of the lumen of the cord. 



The marginal layer gradually thickens by the addition of fibers, some 

 of which grow craniad and some caudad, and most of which soon acquire 

 a myehn sheath. With the appearance of dorsal and ventral nerve roots in 

 connexion with the spinal cord, the marginal zone of white fibers 

 becomes divided into dorsal, lateral, and ventral funiculi. The time of 

 myelination of fibers differs in the several tracts, depending upon the 

 time when they begin to function. Some, such as the pyramidal, become 

 medullated only after birth. 



neural 



■^r — ectoderm 

 neural tube 



Fig. 343. — Drawing showing closure of the neural tube and formation of the neural 

 crest. From pig embryos of : — A, 8 somites; S, 10 somites; C, 11 somites; D, 13 somites. 

 X135. (From Patten's "Embryology of the Pig.") 



Development of Motor Nerves. How nerve and muscle become con- 

 nected with one another has been a much discussed problem in biology. 

 According to Hensen and Kerr the connexion is primary and not second- 

 ary. Nerve and muscle are assumed by them to be, like all the other 

 cells of the body, in protoplasmic continuity which is never completely 

 broken when cells divide. It has, however, been the general opinion of 

 neurologists that the connexion of nerve and muscle is acquired second- 

 arily. According to Francis Balfour chains of cells derived from the 

 central nervous system form the motor nerves. Kupffer, however, 

 claimed that a neurite grows out as a protoplasmic process from a motor 

 ganglion cell and that neuro-muscular connexions are therefore secondary. 

 Many years later Harrison demonstrated experimentally that Kupflfer's 

 assumption is correct. It is this process theory of neurogenesis which 

 underlies Kappers' theory of neurobiotaxis. 



