814 THE NERVOUS SYSTEM 



soft palate, and in the pharyngeal area. In fishes, taste buds are found 

 generally over the buccal cavity and pharynx, and also on the outer 

 surface of the head and branchial region. In some teleosts they may 

 be distributed generally over the external surface of the body (fig. 

 356C). The external distribution of taste buds over the head region 

 occurs also in certain aquatic amphibia. Consequently, the distribution 

 of the epithelial thickenings which give origin to the taste buds varies 

 greatly in different vertebrates. 



3. The Histogenesis of Nervous Tissue 

 a. The Formation of Neurons 



The neurons of the central nerve tube arise from primitive neuroblasts. 

 The primitive neuroblasts in turn take their origin from the cells of the 

 ependymal zone of the nerve tube. The ependymal zone is the layer, two to 

 three cells in thickness, which lines the lumen or neurocoel of the developing 

 tube. Cell proliferation occurs within this zone, and the primitive neuroblasts 

 migrate outward into the more lateral areas. After leaving the immediate 

 confines of the ependymal zone, the neuroblasts presumably begin to differen- 

 tiate into the many peculiar forms of the neurons to be found within the 

 central nervous system. The neurons of the peripheral nervous system arise 

 from cells which migrate from the central nerve tube, and from cells of the 

 neural crests and certain sense placodes. 



1) General Cytoplasmic Changes. The basic physiological functions of 

 irritability and conductivity found in living protoplasm is developed to a 

 high degree in the neuron or essential cellular entity of the nervous system. 

 In consequence, the morphological changes which the simple epithelial cell 

 of the forming neural tube assumes during its differentiation into a neuron 

 is in harmony with these basic functions. One of the morphological changes 

 in the developing neuroblast is the formation of coagulated threads of cyto- 

 plasmic material embedded in a more liquid cytoplasm. These threads are 

 known as neurofibrils, while the more liquid, less-differentiated parts of the 

 cytoplasm are called the neuroplasm. Accompanying the changes which pro- 

 duce the neurofibrils is the formation of another characteristic of neurons, 

 namely, processes or cytoplasmic extensions from the body of the cell (fig. 

 352B). These processes are of two general types, the dendrites and the axon 

 (neuraxis or axis cylinder). Several dendrites are generally present but only 

 one axon is developed. The exact function of the dendrites has been ques- 

 tioned but the possibility is conceded that they function as "the chief receptive 

 organelles of the neuron" (Maximow and Bloom, '42, p. 190), whereas the 

 axon is believed to convey the nerve impulse away from the cell body to the 

 terminal arborizations or teledendria (fig. 352A). The teledendria make physi- 

 ologic contact (i.e., they synapse) with the dendrites of other neurons or they 

 form a specialized relationship with effector cells such as glandular cells or 



