Protoplasm, the Cell, and the Organism - 25 



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Fig. 2-11. Chromosomes from the cells of a newt (Triturus crislatus carnifex). A, compact, or shortened, form 

 of the chromosomes, such as occurs during cell divis : on. Note the pairs (sister chromosomes) that have origi- 

 nated by division. B, a pair of sister chromosomes (the smallest pair of A) after undergoing extension, or elon- 

 gation (diplotene stage of meiosis). Active synthesis of RNA and protein (p. 524) occurs only while the chromo- 

 somes are thus "unfolded." The term "lampbrush chromosomes," often applied to such extended forms, is 

 descriptive of the numerous looplike excrescences that come off from the central core of the chromosomal fila- 

 ment. (Courtesy of H. G. Callan, The University, St. Andrews, Scotland.) 



(Fig. 2-12). Parts of this membrane system 

 display a branching tubular form, but mainly 

 it shows a pattern of numerous parallel 

 double sheets, intricately connected with one 

 another (Fig. 2-13). The total thickness of 

 these double membranous sheets is only 

 about 10 millimicrons, which, of course, pre- 

 cludes resolution by the ordinary micro- 

 scope. 



As will be apparent later (Chap. 8), the 

 endoplasmic reticulum is of great impor- 

 tance. It provides the cell with an extensive 

 system of surfaces (p. 95) upon which various 



enzymes may be localized in orderly pattern. 

 Some of the foci of chemical activity, namely 

 the ribosomes, can be identified in any good 

 electronmicrograph of the endoplasmic retic- 

 ulum. The ribosomes appear in prodigious 

 number, as an array of electron-dense parti- 

 cles lined up in orderly series along the sur- 

 faces of the double membrane sheets (Figs. 

 2-12 and 2-13). Analysis shows that the ribo- 

 somes are composed in large part of RNA- 

 protein material. Apparently they represent 

 focal points for the synthesis of enzymes and 

 other cytoplasmic protein compounds (p. 1 34). 



Fig. 2-10 (facing). Various types of light microscopy are used in studying different features of cell structure. 

 Same cell (Paramecium faursaria) photographed with different techniques; specimen fixed, but not stained; 

 final magnification, X 900 to X 1000. A, ordinary brightfield: light transmitted vertically through the speci- 

 men. B, darkfield: light transmitted more or less horizontally across the specimen. C, polarizing microscope: 

 polarized light transmitted vertically. D and E, phase-contrast microscopy: utilizes changes in the phase of 

 light induced by transmission through the cell structures. F, interference microscopy: proper calibration of this 

 instrument permits calculation of the density of the various cell parts. Note how the sharpness of the different 

 structures varies with the technique; macronucleus, well defined in D, E, and F; refractile crystals, brightly 

 shown in B and C; contractile vacuole (lower end) perhaps best shown in D; symbiotic algae (small round, 

 dark bodies in periphery of cytoplasm) most clearly shown in C and E; gullet (curved structure to right of 

 macronucleus) well defined only in F. (Photographs by Oscar W. Richards, Chief Biologist, American Optical 

 Co.) 



