34 REPRODUCTION AND DEVELOPMENT. 



separated, and the one parent-cell has become two daughter- 

 cells. This process is called direct or amitotic division or 

 fission (Fig. 2). This method of division sometimes occurs 

 with some cells, but it is believed to be far less common than 

 the indirect or karyokinetic method. 



Budding, or gemination: Occasionally this mode of cell- 

 reproduction occurs among the protozoa. A small protrusion 

 of protoplasm appears at the margin of the cell, gradually 

 enlarges, and is finally separated from the parent and becomes 

 independent. 



Segmentation : This consists in the simultaneous breaking 

 up of the parent-cell into a number of small rounded cells, 

 which separate, and in their turn develop to maturity. It 

 occurs among some protozoa, and in human medicine is easily 

 observed in the parasite of malaria, which at the end of its 

 life-period rather abruptly breaks up into from eight to twenty 

 minute hyaline spheres. 



Indirect division, karyokinesis, or mitosis, is the usual and 

 chief mode by which all cells reproduce themselves. The 

 process is accomplished by a series of definite changes, affect- 

 ing especially the chromatin-filaments and centrosomes (Figs. 

 3-8.) The stages of typical karyokinesis may for purposes 

 of description be capitulated as follows : 



1. Resting stage. 



2. Continuous convolution/ "> c!ose s , ke . in ' 



{ 6, open skein. 



3. Segmented convolution. 



4. Equatorial or aster stage. 



5. Division of chromosomes and metakinesis. 



6. Diaster stage. 



7. Double segmented convolution. 



8. Double continuous convolution. 



9. Division of cell-body. 

 10. Resting stage. 



In the intervals between karyokinetic division, or the rest- 

 ing stage f the chromatin of the nucleus is arranged in gran- 

 ules, filaments, or a network, but not in any definite manner. 



The mitotic process begins with an enlargement of the 

 nucleus and an increase of the chromatin ; at the same time 

 the centrosome appears in the body- protoplasm and enlarges. 



