144 



INTRODUCTION TO CYTOLOGY 



In view of the fact that the phenomena of growth, differentiation, 

 reproduction, and inheritance are now known to be intimately bound up 

 with the process of cell-division, it is obvious that a detailed knowledge 

 of this process is an absolute prerequisite to a solution of many of the 

 problems which confront us. In the present chapter the essential fea- 

 tures of vegetative or somatic nuclear division will be described. After a 

 preliminary sketch of the process of mitosis we shall take up in some 

 detail the behavior of the chromosomes and the question of their individ- 

 uality. In the following chapter attention will be devoted to other 

 features of cell- division: the achromatic figure, the mechanism of mitosis, 

 cytokinesis (the division of the extra-nuclear portion of the cell), and the 

 formation of the cell wall. 



SOMATIC MITOSIS 



FIG. 48. Diagram of a typical case of somatic mitosis in plants. 



Preliminary Sketch of Mitosis. The main steps in a typical case of 

 somatic mitosis in plants may be very briefly outlined as follows (Fig. 

 48): 



The chromatic material of the "resting" nucleus, as described in 

 Chapter IV, exists in the form of a more or less irregular reticulum. As 

 the process of mitosis begins this reticulum resolves itself into a definite 

 number of slender threads which represent chromosomes. These in 



w- 



1884, and Heidenhain in 1894 first used the term telokinesis (telophase). Lundegardh 

 (19126) added interphase. The chromosome was named by Waldeyer in 1888. 

 Hermann in 1891 distinguished connecting fibers (central spindle) and mantle fibers. 

 That the halves of each split chromosome go to opposite poles was shown by van 

 Beneden for animals and by Heuser for plants in 1884. The achromatic spindle was 

 first figured by Kowalevsky (1871) and Fol (1873), and first carefully described by 

 Butschli (187506). 



