382 THE FURTHER DIFFERENTIATION OF THE 



These conditions have a bearing on the direction of growth of the 

 peripheral nerve-fibres when they have emerged from the spinal 

 cord. Those fibres which emerge in the ventral region of the cord, 

 continuing to grow along a gradient (and eventually becoming 

 differentiated into axons, since they are growing up the gradient), 

 will accordingly grow to the centre of each muscle-segment, and 

 innervate it. On the other hand, the regions of the septa, between 

 the muscle-segments, will attract the dendrites of the sensory 

 neurons,^ which will then grow to the ends of the muscle-segments 

 (thus providing their proprioceptive innervation), and continue in 

 the septum between the muscle-segments to the skin. 



Both in the sensory and motor systems, therefore, the distribu- 

 tion of the peripheral nerves can be interpreted in terms of gradi- 

 ents : axons growing towards a region of higher rate, and dendrites 

 towards a region of lower rate. Within the central nervous system 

 itself, the same principle can be applied. Experiments of differ- 

 ential susceptibility on the spinal cord indicate that a strip of tissue 

 occupying the ventral mid-line, and forming the so-called keel, 

 has, during late embryonic life, the highest activity-rate at any 

 given level of the cord : this is also proved by the fact that the keel 

 is the site of the most rapid differentiation of neurons in the spinal 

 cord. It is most interesting to find that during this period any axon 

 outgrowths formed in the transverse plane are directed towards the 

 keel. 



In the brain, other centres of differentiation of neurons are the 

 postfacial and prefacial centres, already mentioned, and, further 

 forward, the dimesencephalic, the postoptic and the hemispheric. Up 

 to the early swimming stage, the postfacial centre is the most active, 

 as evidenced by the relative rate at which neurons are differentiated 

 there, compared with the rate in other centres. Correlated with 

 this fact, it is found that the first neurons to become polarised in the 

 dimesencephalic centre send out axons towards and into the pre- 

 facial centre. In a similar way, all over the brain, neurons which 

 are differentiated in the neighbourhood of a centre grow axons to- 



^ The sensory neurons considered here form part of the transient sensory 

 system of Rohon-Beard. They differ, of course, from the sensory neurons of the 

 definitive system in that they are situated in the neural tube instead of the dorsal- 

 root ganglia. Eventually, the Rohon-Beard neurons are superseded by the latter. 



