CHAPTER V 

 THE DIVISION OF THE PROTOPLAST 



The division of one protoplast into two can be seen with Uttle diffi- 

 culty under a microscope, yet the process is one that investigators armed 

 with many techniques have only begun to understand. The significance 

 of much that is seen occurring is evident, but precisely how the various 

 changes are accomplished remains to be discovered. Ordinarily the 

 division of a free cell or a tissue cell results in two cells that have the same 

 structure and capacities as the cell that produced them. In the develop- 

 ment of the body (soma) of a large organism, a long series of such divisions 

 occurs, the many resulting cells eventually becoming unlike in appearance 

 as the soma differentiates. When a reproductive cell — a spore or an egg 

 — is produced, it has all the capacities essential to the development of a 

 complete individual. We are therefore faced with the problem of deter- 

 mining how the highly complex organization of the protoplast can be 

 duplicated w^hen division occurs and, further, just what it is in this organi- 

 zation that enables a spore or an egg to become an adult organism 

 manifesting both general and particular characters of the previous 

 generation. It is the first of these cjuestions that now concerns us. 



For the study of somatic cell division in plants, one may employ large 

 cells that can be kept living in aqueous mediums or paraffin oil while 

 being examined wdth the microscope. The cells of certain filamentous 

 algae {Zygnema, Sphacelaria), the marginal cells of very young leaves 

 (Tradescantia), and the hairs on certain stamens (Tradescantia) and 

 grass stigmas (Arrhenathcrum) have been used very successfully in this 

 way. Young root tips have long been favorite material for somatic- 

 division studies in higher plants, for the regular arrangement of the cells 

 and the large number of divisions visible in one stained section rendei- 

 them almost perfect objects for the purpose (Fig. 37). In animals the 

 dividing eggs of echinoderms and fishes are particularly good (Fig. 38). 

 In later stages of somatic development the embryonic membranes of 

 mammals and the tail fins of tadpoles yield excellent division figures 

 (Fig. 157). The somatic type of division is also well displayed by 

 spermatogonia (but not by spermatocytes). 



It should be reahzed that details of the division process vary wddely 

 in different organisms and tissues. In this chapter we shall confine 

 attention to typical examples. 



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