OUTLINE OF IXDIRECr DIVISIOX OR MITOSIS 47 



bryonic envelopes, and in tumours and other pathological forma- 

 tions, where it is of frequent occurrence. Whether this 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 matura- 

 tion, and by mitotic division 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, Flagel- 

 lates, and Diatoms. We may, therefore, justly regard it as the most 

 general expression of the "eternal law of continuous development" 

 on which Virchow insisted. 



A. Outline of Indirect Division or Mitosis (Karyokinesis) 



The process of mitosis 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, how- 

 ever, graduate into one another and are separated by no well-defined 

 limits. These are : ( i ) The Prophases, or preparatory changes ; 

 (2) the JMctapJiasc, 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 sub- 

 stance, involving both physical and chemical changes. The chroma- 

 tin resolves itself little by little into a more or less convoluted thread, 

 known as the skein (Knauel) or spireme, and its substance stains far 

 more intensely than that of the reticulum (Fig. 19). In some 

 cases there is but a single continuous thread ; in others, the thread 

 is from its first appearance divided into a number of separate pieces 

 or segments forming a segmented spireme. In either case it ulti- 

 mately breaks transversely into a definite number of distinct bodies, 

 known as chromosomes (Waldeyer, '88), which in most cases have 

 the form of rods, straight or curved, though they are sometimes 

 spherical or ovoidal, and in certain cases may be joined together 

 in the 'form of rings. The staining power of the chromatin is now 

 at a maximum. As a rule the nuclear membrane meanwhile fades 

 away and finally disappears. The chromosomes now lie naked in the 

 cell, and the ground-substance of the nucleus becomes continuous 

 with the surrounding cytoplasm (Fig. 19, D, E, F). 



