I 4 6 



THE GROWTH OF THE BRAIN. 



to how far they are to be regarded as variations of a 

 single type. The forms represented by A, E, and D 

 have but a single neuron, that is, are mononeuric, 

 whereas the forms B and C are dineuric, having two 

 neurons. The branching of the neuron near its origin 



FIG. 21. A-D, showing the phylogenetic development of mature 

 nerve cells in a series of vertebrates : a-e^ the ontogenetic 

 development of growing cells in a typical mammal. In both 

 cases only pyramidal cells from the cerebrum are shown. A^ 

 frog ; B, lizard ; C, rat ; D, man ; a, neuroblast without den- 

 drons ; b, commencing dendrons ; <:, dendrons further de- 

 veloped ; d, first appearance of collateral branches ; ^, further 

 development of collaterals and dendrons. (From S. Ramon y 

 Cajal.) 



is, in this connection, a suggestive feature. The above 

 figure (21) shows the manner in which the branchings 

 near the origin of the neuron occur in mononeuric cells, 

 As can be seen, all the branches increase in number 



