156 



NERVOUS TISSUES 



FIG. 171. THE 



SPROUTING OF AN 

 AXON BY A NEU- 

 KOBLAST FROM 

 THE SPINAL 

 CORD OF A FROG 

 EMBRYO. 



From a live spec- 

 imen grown in 

 lymph; the cell 

 body is filled with 

 yolk granules; the 

 protoplasmic proc- 

 ess (axon) is of 

 hyaline appearance 

 and undergoes ame- 

 b o i d movements. 

 (Harrison.) 



work of Harrison and others has established the out- 

 growth view upon a firm basis of observational data. 

 By growing small pieces of the embryonic spinal cord 

 of frogs in lymph, Harrison could observe the cells 

 sprouting an axon process (Figs. 171 and 172). He 

 describes the beginning of a nerve fiber as an outflow 

 of hyaline protoplasm from cells which were situated 

 within the central nervous system. The experiments of 

 Harrison upon frog larvae demonstrate further that the 

 sheath cells of the neurolemma of motor and sensory 

 fibers have their origin in the ganglionic crest, there- 

 fore ectodermal, arid that they are unessential to the 

 formation of the fibrils of the axis cylinder. He ex- 

 cised the dorsal half of the cord, including the neural 

 crest, and observed that in such larvae the fibers of the 

 motor roots did not acquire sheath cells. On the con- 

 trary, when he excised the ventral half of the cord, 

 dorsal root fibers developed normally with a neuro- 

 lemma, but the sheath cells which migrated to the lo- 

 cation where the ventral fibers normally appear were 

 unable to produce 



these fibers in the 

 absence of neuro- 

 blasts in the ven- 

 tral half of the 

 cord. (Anat.Rec., 

 2, 9, 1908.) 



The influence 

 which guides the 

 nerve along its 



proper path is apparently exerted by 

 the tissue which is to be innervated. 

 The essential factors comprising this 

 influence are obscure; they may be of 

 a chemotropic nature. It must be 

 emphasized, however, that the con- 

 nection between a particular nerve 

 and its tissue terminal is made rela- 

 tively early, that is, while the two 

 elements are still spatially relatively 

 closely associated. Probably mechan- 

 ical stimuli, inducing thigmotropic 

 reactions, also play an important role 

 in determining the path of a nerve 



FIG. 172. THE SPROUTING OF AN 

 AXON BY A NEUROBLAST FROM THE 

 SPINAL CORD OF A FROG EMBRYO. 



Two views of the same nerve fiber, 

 grown in lymph, taken twenty-five 

 minutes apart. (Harrison.) 



