CELLS AND TISSUES 



13 



elements (including the wall) rather than in any conspicuous structural 

 changes in the protoplasm itself that cell differentiation is rendered 

 visible in the case of plants. Apart "from differences in shape, amount of 

 vacuolar material, accumulated food, and other products of differentia- 

 tion, protoplasts performing widely different functions may appear much 

 alike. 



Structural differentiation in connection with division of labor is very 

 striking in the protoplasm of animal cells," which are destitute of such 

 ■walls as plant cells possess. The muscle cell shows fine longitudinal 



((if 



VMS' 

 V 



1 ) 



J 



i 



B 



Fig. 8. — Differentiated cells from vascular plants. A, wood fiber with thickened wall. 

 B, C, portions of tracheids with spiral and annular thickenings. D, pitted tracheid. 

 E, portion of sieve tube with adjacent companion cells. F, face view of sieve plate shown 

 in section in E. 



fibrils, which are in some way concerned with the cell's power of contrac- 

 tility. In certain muscles these fibrils show transverse membranes 

 at regular intervals, and these so correspond in adjacent fibrils that the 

 muscle has a transversely striped appearance. 



The nerve cell (Fig. 9) typically possesses a single unbranched pro- 

 longation (axon) and one or more others (dendrites) which often become 

 very elaborately branched, especially in the ganglion cells of the spinal 

 cord and brain. In fixed preparations the cytoplasm of the nerve cell 

 contains fine "neurofibrils"^^ and also granules of chromatic "Nissl 

 substance." 



^1 For detailed descriptions of the cytological features of animal cells of many 

 types, see Special Cytology, ed. by Cowdry (1932); see also Heidenhain (1907, 1911). 



12 It has been reported that in healthy living cells the neurofibrils cannot be 

 detected and probably represent coagulation artifacts (Matsumoto, 1920; Lewis and 

 Lewis, 1924; de Mouhn, 1923), but other investigators (Cowdry, 1914, 1928; Bozler, 

 1927; Boeke, 1926; Parker, 1929a6) are convinced that they correspond to real 

 differentiations in the cell. 



