MORPHOLOGY OF EYE MUSCLE NERVES 19 



'07) has modified this view by assuming that mitotic cells form 

 the first anlage of the nerve and the development of the nerve 

 proceeds by cell multiplication. Not every fiber, however, arises 

 from a series of cells, but a series of cells produces a large number 

 of fibers. 



3. The third theory of nerve histogenesis, first formulated by 

 Kupffer and Bidder ('57), but finally developed as the neuro- 

 blast theory by His ('79), holds that the nerve fiber develops as 

 the process of an embryonic ganglion cell. Even before this 

 theory had been advanced it had been noted by Remak ('55) 

 that nuclei are absent in embryonic nerves. His ('79, '88, '90, 

 '93, '04) developed this process theory of Kupffer and Bidder 

 as the 'neuroblast theory' of nerve histogenesis, according to 

 which nerves are formed as processes of special cells or 'neuro- 

 blasts.' The first outgrow^th of these cells forms the axis cylin- 

 der and the cell becomes a ganglion cell. A single process is 

 formed by the neuroblasts of the neural tube, while two are 

 produced by the neuroblasts of the peripheral ganglia. Other 

 branched processes formed later become the dendrites of the 

 ganglion cells. Thus every single nerve fiber represents a single 

 and definite cell of origin which constitutes its genetic, nutritive 

 and functional center. These nervous units do not unite with 

 each other in protoplasmic continuity, but a transfer of impulses 

 occurs without continuity. This view has been supported by 

 von Koelliker ('86, '91, '92, '00, '05), Sagemehl ('82), Ram6n y 

 Cajal ('90-'07), von Lenhossek ('90-'03), Retzius ('93), Neal ('98, 

 '03, '12), Gurwitsch ('00), Harrison ('01-'12), Bardeen ('03), 

 Lewis ('06, '07), Dohrn ('07), Belogolowy ('10) and Burrows ('11). 



Waldeyer ('91) has still further developed the neuroblast theory 

 as the neuron theory, according to which the nervous system, 

 both central and peripheral, consists of independent functional 

 units — cells or neurons — which never become fused or united 

 but transmit impulses by transfer or induction from one neuron 

 to another. 



Later investigators have advocated various modifications of 

 these three theories, but not hypotheses that may be sharply 

 distinguished from the three theories mentioned above. They 



