CHAPTER III 



HISTOGENESIS OF THE NERVOUS SYSTEM 



Early Stages in the Differentiation of the Neural Tube. Hardesty (1904) 

 has given a good account of the early development of the spinal cord in the pig. 

 At first the neural plate consists of a single layer of ectodermal cells (Fig. 19, A). 

 These proliferate and lose their cell boundaries. When the neural tube has 

 closed its wall is formed of several layers of fused cells a syncytium bounded 

 by an external and an internal limiting membrane (Fig. 19, B, C). The syn- 

 cytium now becomes more open and sponge-like in structure. The nuclei are 

 so arranged that three layers may be differentiated: (1) an ependymal layer, 

 (2) a mantle layer, with many nuclei, and (3) a marginal or non-nuclear layer. 

 The ependymal layer is represented by a row of elongated nuclei, among which 

 are found the large mitotic nuclei of the germinal cells. 



These germinal cells divide and give rise to ependymal cells, and to the indif- 

 ferent cells of the mantle layer. Through division of the latter spongioblasts 

 and neuroUasts are formed. From the former comes the neuroglia or supporting 

 tissue of the nervous system, while from the latter are derived the nerve-cells 

 and fibers. 



The Development of the Neuron. A neuron may be defined as a nerve- 

 cell with all its processes; and each is derived from a single neuroblast. From 

 the pear-shaped neuroblast a single primary process grows out, and this be- 

 comes the axis-cylinder of a nerve-fiber (Fig. 20). Other processes which de- 

 velop later become the dendrites. The primary process, or axon, grows into the 

 marginal layer, within which it may turn and run parallel to the long axis of the 

 neural tube as an association fiber; or it may run out of the neural tube in a ven- 

 trolateral direction as a motor axon. In this way the motor fibers of the cere- 

 brospinal nerves are laid down. The axis-cylinder of each represents a process 

 which has grown out from a neuroblast in the basal plate of the neural tube. 



Development of Afferent Neurons. The sensory or afferent fibers of the 

 spinal nerves take origin from neuroblasts which are from the beginning out- 

 side the neural tube. These neuroblasts are derived from the neural crest, a 

 longitudinal ridge of ectodermal cells at the margin of the neural groove, where 

 this becomes continuous with the superficial ectoderm. At first in contact with 



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