366 



Special Vertebrate Organogenesis 



the neuron, the well-known impairment of 

 nerve growth in thiamine deficiency (beri- 

 beri) is observed only if the cell bodies are 

 bathed by the deficient medium (in vitro), 

 even though the rest of the nerve, including 

 the growing tips, lies in normal medium 

 (Burt, '43a). 



The correlation of cytological, biochemi- 

 cal and physiological data for normal and 

 deficiency states of the nervous system is 

 making encouraging progress (see p. 376). 

 One of the major tasks in these studies which 

 remains is to distinguish clearly between 

 the relative contributions of the metabolic 

 machinery of the neuron to its growth (i.e., 

 protoplasmic reproduction) on the one hand, 

 and to its functional activity on the other; 

 a task rendered more difficult by the fact 

 just outlined, that having reached a sta- 

 tionary size, a neuron keeps "growing" with- 

 out change of total mass. 



DEVELOPMENT OF GROUP RELATIONS 



Fasciculation. Outside the gray matter and 

 the terminal arborizations, nerve fibers 



of pioneering fibers, the routing is achieved 

 not by "attraction" but by a form of con- 

 tact guidance. When a nerve source and a 

 peripheral organ, e.g., a limb, are implanted 

 at some distance from each other in a loose 

 connective tissue bed (e!g., the dorsal fin 

 of an amphibian larva), a strong nerve cable 

 soon develops between them (Fig. 136) in 

 the following manner (Weiss, '50a). Pio- 

 neering fibers from the nerve center invade 

 the surroundings. Those that happen upon 

 the limb and succeed in connecting with its 

 tissues thereby become somehow adhesive 

 for other nerve fibers growing out subse- 

 quently; older fibers thus become guides to 

 the limb for younger ones, endowing them, 

 in turn, with adhesiveness, and so forth, in 

 a sort of chain reaction. It is not known what 

 eventually terminates the agglomeration; 

 perhaps fasciculation ceases automatically 

 as soon as peripheral saturation (see p. 368) 

 has been reached, that is, when further 

 newcomer fibers no longer find functional 

 attachments, hence, do not become adhesive. 

 The lessons of nerve anatomy which show 

 that nerve fibers of identical functional 



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Fig. 136. Fasciculation. Nerve cable that has formed between a fragment of spinal cord (left) and a limb 

 (right), both deplanted to the dorsal fm of a urodele larva (frontal section, showing the loose connective 

 tissue of the fin between the two borders of epidermis). (From Weiss, '50a.) 



rarely appear solitary. They commonly as- 

 sociate with other nerve fibers to form bun- 

 dles, as well as with the Schwann cells 

 and connective tissue cells. The grouping of 

 fibers into bundles seems to come about in 

 two ways: {a) by a primary tendency of 

 younger fibers to follow the course of older 

 fibers in close application of one to the 

 other ("fasciculation"); and {b) by the 

 secondary gathering of small groups into 

 larger assemblies through the formation of 

 comraon connective tissue sheaths. 



Primary fasciculation is not a chance event 

 but a systematic device to route a sufficient 

 number of nerve connections toward a desti- 

 nation in need of them. As in the outgrowth 



properties (e.g., fibers subserving pain or 

 special sensations) often course together, 

 both in peripheral nerve trunks and central 

 funicidi, indicate an element of selectivity 

 in the process of fasciculation. Young fibers 

 of a given kind would apply themselves 

 preferentially to older fibers belonging to 

 the same, rather than to another, category. 

 Some experimental proof for "selective fasci- 

 culation" by contact affinities may be seen in 

 the observation that superniimerary Mauth- 

 ner's fibers of the amphibian brain run along- 

 side the normal Mauthner's fibers of that side 

 much more frequently than could be expected 

 on mere chance; and in the further observa- 

 tion that transected longitudinal fiber tracts 



