THE NERVE ELEMENTS. 



the typical neuroblasts the nucleus is large, and the 

 cytoplasm seen only as an accumulation at one pole, 

 being drawn out into a slender thread which represents 

 the beginnings of the neuron. It has been shown by 

 His, for example, that the mass of the neuroblast was, 

 for a time after its formation, approximately equal to 

 the mass of the parent ger- 

 minal cell ; the difference 

 being that in the germinal 

 cell the cytoplasm sur- 

 rounded the nucleus on all 

 sides, whereas in the neuro- 

 blast it was extended to form 

 an embryonic fibre. 



This relation enables us to 

 take a germinal cell as the 

 unit of volume from which 

 to calculate the subsequent 

 changes in size. Moreover, 

 like the term cell, the term 

 neuroblast in this discussion 

 will be used to mean not 

 only the nucleus with its 

 surrounding cytoplasm, but 

 also that portion of the 



cytoplasm which forms the FlG . 24 ._ Portion of developing 

 young neuron. As the neuro- 

 blast develops, a striking 

 change is caused in the 

 cell-body by the apparently 



great increase in the cytoplasm, but this is really a small 

 matter as compared with the increase in substance repre- 

 sented by the elongation and enlargement of the neuron. 

 In this latter change the increase in the length of the 

 neuron is the more important than the increase in dia- 



medullary tube seen in frontal 

 section. Human, X 1,100 

 diameters (His). , germinal 

 cell ; N, neuroblasts. 



