148 THE GROWTH OF THE BRAIN. 



on this distal end arouses an impulse which passes 

 towards the cell-body, through it, and out by way of 

 the other neuron, which in the case of these cells 

 extends, as the dorsal nerve root, into the substance of 

 the spinal cord, this root being the pathway by which 

 the incoming impulses reach the central system. As a 

 consequence of stimulation the cells themselves are 

 changed, and it is therefore to be expected that the 

 impulse leaving the cell would be different from that 

 which entered it. Returning to the matter of form, the 

 question arises whether the mononeuric cells, like those 

 in the cortex (Fig. 21), are similar in fundamental struc- 

 ture to the more primitive dineuric cells, and have been 

 derived from them. In favour of the view that they are 

 thus similar, and derived, a few facts may be adduced. 

 These ganglia in lower vertebrates are made up of 

 dineuric cells, like those in Fig. 22. This form is 

 assumed by the ganglion cells of mammals during their 

 early development. In these latter, however, it is not 

 permanent, for as growth continues the points of 

 entrance of the neurons into the cell-body approximate 

 until they come together, so that the cell-body finally 

 stands at one side of the passing neuron. The change 

 does not, however, stop here. From the neuron the cell 

 becomes still further separated in such a way that it 

 appears to stand at the end of a single stem, a stem 

 which by its development is composed of those portions 

 of the two neurons which were closest to the cell-body, 

 and by virtue of its origin this stem should therefore 

 contain two pathways. 



The transformation can be beautifully seen in the 

 developing Gasserian spinal ganglion of the guinea-pig. 

 Here the cells when first formed are dineuric, as in the 

 fish, and only gradually become modified to the extreme 

 forms shown in the figure. It has been pointed out that 



