75 2 



A MANUAL OF PHYSIOLOGY 



it by any means so easy to understand as on the neuron hypothesis 

 such facts as the strict limitation of Wallerian degeneration to the 



FIG. 305. 



a e shows the development of the pyramidal nerve-cells of the cerebral cortex 

 in a typical mammal : a, neuroblast with commencing axon ; b, dendrites ap- 

 pearing ; d, commencing collaterals. A D shows the different degree of com- 

 plexity in the fully-developed pyramidal cells in different vertebrates : A, frog ; 

 B, lizard ; C, rat ; D, man (Donaldson, after Ramon y Cajal). 



boundaries of the neurons directly affected, or the strict limitation 

 of the silver reduction in Golgi preparations to single neurons. 

 It is, of course, true that the simplicity and order introduced by the 



FIG. 306. 



Cells from the Gasserian ganglion of a developing guinea-pig. The originally 

 bipolar cells are seen changing into cells apparently unipolar. The same process 

 occurs in the cells of the spinal ganglia (Van Gehuchten). 



neuron hypothesis into our conceptions of the nervous conduction 

 paths by no means prove its accuracy. Yet they are reasons for 

 not lightly abandoning it. 



