Nervous System 



377 



easily distinguished by their inclusions, dif- 

 ferent gland cells by their secretions; but in- 

 dividual nerve cells, judged by their shapes, 

 whose normally great variability is further 

 exaggerated under experimental conditions. 



within each class, which must be postulated 

 on functional evidence (see below, p. 384), 

 for instance, between motor cells innervating 

 different muscles or between neurons sub- 

 serving different sensory modalities. The 



^:X^ 



c I 



rt 



ih ',, 



^i'. 



^rr^ 



^a^ 



Fig. 141. Diagrammatic frontal sections of the spinal cord of chick embryos of 4 days {A), 5 days (B) and 

 8 days (C) , showing the regionally differing formation of the motor columns from a morphologically rather 

 uniform condition. In B, degeneration of cells in the cervical region (c, black circles), and centripetal emi- 

 gration of cells in the thoracic and sacral regions {th) have set in, resulting in the distribution shown in C. 

 (From Hamburger and Levi-Montalcini, '50.) 



cannot always be so readily identified as to 

 the precise type to which they belong. Large 

 motor cells, Rohon-Beard cells, spinal gan- 

 glion cells, commissural cells, and a very few 

 others are sufficiently distinctive to be used 

 as cytological indicators for the respective 

 neuron types. But there are no corresponding 

 microscopic signs for the finer distinctions 



only individual cell that can be strictly 

 identified as such is Mauthner's neuron, of 

 which there is in many species only a single 

 pair, situated at the otic level of the hind- 

 brain of lower vertebrates, and which can 

 be recognized by its giant size. For the rest, 

 one must rely on general morphological 

 group criteria, such as cross-sectional con- 



