348 



Special Vertebrate Organogenesis 



terms. Much of the current research in neuro- 

 genesis is still in this descriptive phase, even 

 though, in order to get the necessary data, it 

 makes extensive use of experiments. The 

 mode of the formation of a nerve fiber is 

 a case in point. At the turn of the century, 

 there were two opposing schools of thought 

 given to different interpretations of incon- 

 clusive static observations. The one main- 

 tained that the axis cylinder is produced 

 in fractions by tandem chains of peripheral 

 cells, which are then secondarily joined to- 

 gether into a single strand, drainpipe fashion; 

 while the other contended the axon to be a 

 protoplasmic sprout of a single central neuro- 

 blast cell. The ingenious experimental feat 

 of Harrison ('07a, '10) in isolating the sup- 

 posed neuroblasts in extraneous media devoid 

 of peripheral cells to test whether or not 

 they could still form axons, settled the issvie: 

 they could. A descriptive datum had been 

 ascertained by an experimental method. 

 Then, passing on to explore the reaction of 

 cells in vitro to solid fiber substrata, Harrison 

 ('14) carried his research into the strictly 

 analytical sphere, where one examines why 

 things happen as they do — in the given case, 

 why the nerve fiber follows one course rather 

 than another. The Hows and Whys of our 

 questions are thus intimately related and 

 often enough blend into one. With this in 

 mind we may now attempt to carve out some 

 specific neurogenetic questions from the bodv 

 of neurological data presented above. Evi- 

 dently, this can only be a crude and frag- 

 mentary sample. 



Why and where does the axon arise from 

 the neuroblast? What causes its elongation? 

 What gives it its course? Do the trunks and 

 branches of the mature nerves reflect the 

 orientation of early outgrowth? Is that out- 

 growth strictly oriented or is it haphazard, 

 followed by selective abolition of unsuccess- 

 ful connections? What determines deflections 

 or other changes of course? What causes 

 branches to arise, and where? Are tissues 

 flooded with nerve fibers, or is admission 

 selective? If the latter, how is invasion 

 held in check? And is penetration tan- 

 tamount to functionally effective innervation? 

 What causes the association of sheath cells 

 and nerve fibers, and what is the mechanism 

 of myelin formation? How do fibers group 

 into bundles — by active aggregation or by the 

 enveloping action of connective tissvxe? And 

 what determines the places and proportions 

 in which the various tissue elements combine 

 to form nerves? How does it happen that 



fibers of similar function are often grouped 

 together, and how do they each reach their 

 appropriate destinations? Or do they? And, if 

 not, how can central functions fail to be 

 confused? How does a nerve fiber gain in 

 width, and what decides its final caliber? 

 And does it change with body growth? What 

 controls the number of fibers available for a 

 given area — size of the source, frequency of 

 branching, overproduction followed by ter- 

 minal screening, or all of these? And if the 

 size of the source is a factor, what determines 

 it? This points us to the centers. 



How does the neural plate transform into 

 primordia of brain, spinal cord, and ganglia? 

 How does it grow? How do its cell groups 

 specialize for their respective formative tasks, 

 how early, in what places and what se- 

 quence? What makes them divide or cease to 

 divide? What causes them to migrate and in 

 what directions, and what to assemble in 

 defined locations? What sets the numbers 

 and quotas of the different neuron types, and 

 adjusts them to the functional needs of the 

 individual? How do they achieve selective 

 interconnections on which their later fimc- 

 tioning will depend? And which ones of these 

 are really relevant to the specific patterns, 

 rather than just the general execution, of 

 central functions? What provides the neuron 

 population with the proper contingent of 

 supportive, protective, and nutrient cells and 

 structures of other origins, in varying com- 

 binations according to the local needs? And 

 how much interdependence and interaction 

 in erowth and differentiation is there between 

 different central regions before and after they 

 have become segregated? If there are inter- 

 actions, what is their nature and how are 

 thev transmitted? Does exercise and practice 

 have a constructive, or at least modifvinp'. 

 effect on central Tiathwavs or central size? 

 Are fluctuating neripheral demands taken in- 

 to account in the development of centers, 

 and, if so, bv what means? Can growing 

 centers adjust to lesions or deformation, 

 and how — ^bv ree:eneratinn, comnensatorv 

 e-rowth. or substitutive functional correc- 

 tions? And can the development of overt 

 behavior be correlated with, or even ex- 

 plained by, the stepwise emergence of neural 

 apr)aratnses? 



Specific questions like these, rather than 

 noncommittal ereneralities about "the de- 

 velopment of the nervous system being a 

 matter of metabolic processes, gradient fields 

 and enzymatic reactions," are effective guides 

 to useful research. 



