MITOTIC DIVISION OF CELLS. 



Centrosome / 



withjA 



centrioles /> 



"r 



1 

 1 



i 



Nucleus 



n 



rate are equal, the tissue or organ is in a state of equilibrium. As soon as the 

 death-rate exceeds the multiplication rate, decay and atrophy set in ; and when 

 the decay and atrophy have proceeded to such an extent that an important tissue 

 or organ can no longer perform its proper functions, general death ensues. 



General decay and death are, therefore, the natural results of the loss of 

 multiplication power of the cells of the body, but life may persist after multiplica- 

 tion power is lost, so long as the cells last produced retain their capabilities, and 

 death may result whilst multiplication power 

 of the cells is retained, if the newly produced 

 cells are incapable of performing their proper 

 functions. Nevertheless, speaking generally, 

 it may be said that cell multiplication is 

 a vital necessity, and it takes place in two 

 ways (1) by amitotic and (2) by mitotic division 

 of pre-existing cells. 



Amitotic Division. The phenomena of 

 amitotic division, so far as they are known, are 

 much simpler than those of mitotic division. 

 First the nucleus is constricted and divided ; 

 then the cell body is constricted and divided, 

 and two similar daughter cells, each half the 

 size of the mother cell, are produced. The part 

 played by the centrosome during the process 

 is not definitely known, but each daughter cell 

 eventually possesses a centrosome. The appar- 

 ently simple process of amitofcic division occurs 

 at some periods of growth, and the more com- 

 plicated process of mitotic division at other 

 periods, but the laws which govern the alterna- 

 tions are unknown. 



Mitotic Division ; Mitosis, or Karyokin- 

 esis. Mitotic or karyokinetic division is not 



Only the more complicated, but it appears also FlG - 4. SCHEMA OP ANIMAL CELL IN EARLY 



to be the more important form of cell division. PART OF PROPHASE OF HoMOTYPE MlTOSIS ' 



It takes place in all rapidly growing tissues, 



especially in the embryonic and foetal stages 



of life, and it is the main form of cell division 



which occurs in the earliest embryonic periods. 



There are, however, two forms of mitosis, the 



homotype and the heterotype. Of the two, 



homotype is so much the more common that it 



may be looked upon as the ordinary form, for 



heterotype mitosis appears to be limited to one 



of the two cell divisions which occur during 



the maturation of the germ cells, and to 



some of the cell divisions which are associated 



with the production of malignant tumours. 



Homotype Mitosis. The phenomena of 

 homotype mitosis occur in four phases, (1) 

 the prophase, (2) the metaphase, (3) the anaphase, and (4) the telophase. 



The Prophase. During the prophase both the centrosome and the nucleus 

 undergo very obvious transformations. 



The centrosome and its contained centriole divide into two parts, of which one 

 passes to one pole and the other to the opposite pole of the nucleus. 



The nuclear transformations concern the nucleolus, the chromatic substance, 

 and the nuclear membrane. 



The nucleolus disappears. In some cases it passes from the nucleus into the 

 cytoplasm, where it breaks up ; in other cases the details of its disappearance are 

 entirely unknown. 



FIG. 3. SCHEMA OF ANIMAL CELL IN 

 RESTING STAGE. 



Daughter 

 centrosome x^ 



Nucleus with 



chromatic/' 

 substance in 

 skein form! .-.' 



Achromatic 

 spindle 



Daughter centrosome 



Achromatic 

 spindle 



Chromosomes L'l^-- 

 at equator '/ ' 

 of spindle y/:; 



V- 



FIG. 5. SCHEMA OF ANIMAL CELL AT COM- 

 PLETION OF PROPHASE OF HOMOTYPE 

 MITOSIS. 



