OUTLINE OF INDIRECT DIVISION 65 



view be well founded or not, it is certain that in all the higher and in 

 many of the lower forms of life, indirect division or mitosis is the 

 typical mode of cell-division. It is by mitotic division that the germ- 

 cells arise and are prepared for their union during the process of 

 maturation, and by the same process the oosperm segments and gives 

 rise to the tissue-cells. It occurs not only in the highest forms of 

 plants and animals, but also in such simple forms as the rhizopods, 

 flagellates, and diatoms. We may, therefore, justly regard it as the 

 most general expression of the "eternal law of continuous develop- 

 ment" on which Virchow insisted. 



A. Outline of Indirect Division or Mitosis (Karyokinesis) 



In the present state of knowledge it is somewhat difficult to give a 

 connected general account of mitosis, owing to the uncertainty that 

 hangs over the nature and functions of the centrosome. For the pur- 

 pose of the following preliminary outline, we shall take as a type 

 mitosis in which a distinct and persistent centrosome is present, as 

 has been most clearly determined in the maturation and cleavage of 

 various animal eggs, and in the division of the testis-cells. In such 

 cases the process involves three parallel series of changes, which affect 

 the nucleus, the centrosome, and the cytoplasm of the cell-body 

 respectively. For descriptive purposes it may conveniently be divided 

 into a series of successive stages or phases, which, however, graduate 

 into one another and are separated by no well-defined limits. These 

 are: (i) The Pjvp/iascs, or preparatory changes; (2) the Metapliase, 

 which involves the most essential step in the division of the nucleus ; 

 (3) the Anaphases, in which the nuclear material is distributed ; (4) the 

 Telophases, in which the entire cell divides and the daughter-cells are 

 formed. 



I. Prophases. — {a) The Nucleus. As the cell prepares for division, 

 the most conspicuous fact is a transformation of the nuclear substance, 

 involving both physical and chemical changes. The chromatin-sub- 

 stance rapidly increases in staining-power, loses its net-like arrange- 

 ment, and finally gives rise to a definite number of separate intensely 

 staining bodies, usually rod-shaped, known as cJiromosouies. As a rule 

 this process, exemplified by the dividing cells of the salamander-epi- 

 dermis (Fig. i) or those of plant-meristem (Fig. 2), takes place as fol- 

 lows. The chromatin resolves itself little by little into a more or less 

 convoluted thread, known as the skein (Knauel) or spireme, and its sub- 

 stance stains far more intensely than that of the reticulum (Fig. 25). 

 The spireme-thread is at first fine and closely convoluted, forming the 

 "close spireme." Later the thread thickens and shortens and the 



