THE BIOLOGY OF THE CELL SURFACE 



substances in solution. Indeed, with respect to the nucleo- 

 cytoplasmic relations, the so-called resting stage of the 

 nucleus Is as important as the stage of division. 



During the stages of nuclear division there also occur 

 changes in the cytoplasm, as changes in refraction, in 

 dispersion of granules, changes within the granules them- 

 selves. The fact that these changes can be correlated 

 with the stages of nuclear behavior suggests that chemical 

 syntheses and analyses in the living cell are in part at 

 least controlled by the movement of substances in solu- 

 tion to and from the cytoplasm out of and into the nucleus. 

 To this point I return later. 



As has been said above, the cytoplasm is composed of an 

 inner core immediately surrounding the nucleus and, 

 external to this, an ectoplasmic region extending to the 

 cell-membrane. In this thus differentiated area of extra- 

 nuclear protoplasm formed bodies of different size are 

 suspended. By determining what in this extra-nuclear 

 mixture of fluid and suspended bodies is indispensable to 

 the life of the cell, we may derive a definition of cytoplasm. 



The suspended bodies are chiefly: droplets of oil, yolk, 

 small formations known as Golgi-bodies, granules called 

 mitochondria and chromidia; further, starch, crystals of 

 various kinds, secretion-granules, excretory products. But 

 these cytoplasmic inclusions are not to be taken for the 

 cytoplasm itself, as is strongly indicated by the following 

 facts. First, animal cells vary with respect to the content 

 of cytoplasmic inclusions: not all cells contain them all and 

 some even under high power of the microscope appear to 

 contain none. Second, in many cases the inclusions occupy 

 no constant position in cells: thus yolk generally present in 

 eggs is variously distributed. Third, and most important, 

 portions of eggs devoid of all cytoplasmic inclusions having 

 been fertilized develop as do whole eggs with all inclusions 

 present. Consider the egg of the marine worm, Chaetop- 



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