18 



INTRODUCTION PKOTOPLASM CELL 



monera (Haeckel), but not perceptible under our present means of 

 search and observation. This is the position urged by one of the leading 

 physiologists and histologists (E. A. Schaefer) of our day, following 

 Spencer of the preceding generation. 



However, in histology we need be concerned only with the derivation 

 of cells from preexisting cells. This proceeds in one of two ways: (a) 

 direct, amitotic, or akaryokinetic ; and (b) indirect, mitotic, or karyokin- 

 etic. The difference between the two inheres in a difference in behavior 

 on the part of the nucleus (or karyon). Comparative studies of the 



FIG. 24. SUCCESSIVE STEPS IN AMITOTIC DIVISION IN TENDON CELL OF NEW-BORN 



MOUSE. 

 (After Nowikoff. X 800.) 



lower groups of animals and plants have revealed a fairly complete series 

 of intermediate stages. On the basis of these facts it is believed by some 

 (e.g., Strasburger) that amitosis is the primitive method of cell 

 multiplication, mitosis the derived or more highly specialized type. 

 Others regard amitosis as the derived, not the primitive form of 

 division. 



Cell division is presumably due to the fact that the area of the 

 surface increases as the square, the volume as the cube, of the diameter. 

 In consequence, the periphery becomes more favorably placed with respect 

 to the nutritive medium than the more central portions. The time then 

 arrives when the center must suffer nutritive want or when the nucleus 

 becomes unable to exert its trophic functions at the distance of the ad- 



