8 



INTRODUCTION TO CYTOLOGY 



of materials between nucleus and cytoplasm, so that a certain proportion 

 of nuclear and cytoplasmic substances (the " nucleoplasmic ratio") must 

 be maintained if these processes are to continue. The part which any 

 single spherical nucleus can play is strictly limited by the simple fact 

 that its surface, through which the interchanges occur, does not increase 

 at the same rate as does its volume during growth. The further growth 

 of the protoplasmic mass therefore requires a relative increase in the 

 nuclear surface. This is sometimes accomplished by a change in the 

 shape of the nucleus, but the almost universal method is by nuclear divi- 

 sion, whereby the nuclear surface is increased without a corresponding 

 change in volume. This permits further growth up to the point at which 



the critical ratio is again reached, 

 whereupon the process is repeated. 

 The same principle is applicable 

 to the relation between the proto- 

 plasmic mass and its environment. 

 The size which a globular mass of 

 protoplasm can attain through an 

 increase in the actual amount of its 

 substance is limited by a critical ratio 

 between its volume and the amount 

 of surface through which interaction 

 with the external environment is 

 carried on. The maintenance of the 

 volume-surface ratio necessary to a 

 proper metabolic equilibrium in the 

 growing mass is secured in several 

 ways. The mass may simply divide 

 into two, thus increasing the surface 

 without immediate increase in vol- 

 ume, and incidentally multiplying 

 the number of individuals (Protozoa 

 and Protophyta). A second method is by change of shape. This is 

 well exemplified in coenocytic plants, such as Vaucheria (Fig. 4), Mucor, 

 and the myxomycetes, in which large masses of protoplasm expose an 

 extensive surface by assuming a flat, spreading form (myxomycetes) or 

 by developing filamentous branching bodies (coenocytic algae and 

 phycomycetes). At the same time the nucleoplasmic ratio is maintained 

 by repeated nuclear division, the numerous nuclei being either relatively 

 fixed in position (Caulerpa) or free to move about with the flowing 

 cytoplasm (Brijopsis, phycomycetes, myxomycetes). 



The formation of partitions and the division of the nuclei show various 

 degrees of correlation in different organisms. The two processes may be 

 quite independent, in which case the compartments (cells) contain vary- 



FiG. 4. — V, portion of ccenocytic body of 

 Vaucheria; nuclei dark and plastids in out- 

 line. C, portion of semi-ccenocytic body of 

 Cladophora. 



