BASIC DEVELOPMENTAL FEATURES 815 



muscle fibers (fig. 352A). The neurofibrils extend into the cell processes. The 

 precise relationship of the neurofibrils to conduction and transmission of nerv- 

 ous impulses is unknown. (Note: The formation of the sheaths surrounding the 

 nerve fiber is described on page 819.) 



2) Nuclear Changes. Associated with the changes in the cytoplasm men- 

 tioned above are alterations of the nucleus. One of the striking features of 

 nuclear change is that it enlarges, and becomes vesicular, though the basichro- 

 matin remains small in quantity. The nucleolus experiences profound changes, 

 and is converted from a homogeneously staining body into a vacuolated struc- 

 ture in which the desoxyribose nucleic acid is irregularly localized along the 

 edges. Contemporaneous with the nucleolar changes there is a "marked produc- 

 tion of Nissl substance in the cytoplasm" (Lavelle, '51, p. 466). Accompany- 

 ing the changes in the nucleus is its loss of mitotic activity, although a centro- 

 some is present in the cytoplasm. All neuroblasts, however, do not lose their 

 power of division; only those which start to differentiate into neurons. During 

 embryonic life many potential neurons remain in the neuroblast stage and these 

 continue to proliferate and give origin to other neuroblasts. Shortly after birth 

 or hatching this proliferative activity apparently ceases, and the undifferen- 

 tiated neuroblasts then proceed to differentiate into neurons. 



3) Growth and Development of Nerve-cell Processes. The early neuroblasts 

 of the central nerve tube are at first apolar, that is, that do not have distinct 

 processes. These apolar cells presumably transform in unipolar and bipolar 

 varieties of neuroblasts. The unipolar cells have one main process, the axon, 

 and the bipolar cells have two processes, an axon and a dendrite. From 

 these two primitive cell types multipolar neurons arise having several dendrites 

 and one axon (fig. 352B). 



As the nerve-cell process begins to develop, a small cytoplasmic extension 

 from the cell body occurs. To quote directly from Harrison ('07), p. 118, 

 who was the first to study growing nerve-cell processes in the living cell: 

 "These observations show beyond question that the nerve fiber develops by 

 the outflowing of protoplasm from the central cells. This protoplasm retains 

 its amoeboid activity at its distal end, the result being that it is drawn out 

 into a long thread which becomes the axis cyhnder. No other cells or living 

 structures take part in the process. The development of the nerve fiber is 

 thus brought about by means of one of the very primitive properties of living 

 protoplasm, amoeboid movement, which, though probably common to some 

 extent to all cells of the'embryo, is especially accentuated in the nerve cells at 

 this period of development." The distal end of a growing nerve fiber has a 

 slight enlargement, the "growth cone" or "growth club" (fig. 352C). The 

 conclusions of Harrison on growing nerve fibers in tissue culture were sub- 

 stantiated by Speidel ('33) in his observations of growing nerve fibers in the 

 tadpole's tail. 



Many different shapes of cells are produced during the histogenesis of the 



