154 



MAN, THE ANIMAL 



takes place, numerous cell divisions occur; and 

 when these are followed backward, one comes to 

 the beginning, a single cell. There Is thus an in- 

 teresting similarity between the simple ameba 

 and the fertilized egg from which grows the 

 mature animal. 



Figure 53. Nerve cells in black from the human brain. The 

 one with branches on all surfaces is from the cerebrum while 

 the other is from the cerebellum. The remaining nerve cell 

 shows the cell-body only. There is a prominent nucleus and 

 triangular bodies in the cytoplasm which often undergo change 

 in shape in disease. 



There are just two structures which all animals 

 with a differentiated nervous system possess in 

 common: nerve cells and synapses. In the study 

 of all other features of protoplasm, the biological 

 unit, the cell, has been a necessary starting place. 

 What are nerve cells? 



This question may seem to be a strange intro- 

 duction to a discussion of the law of sensation In 



