THE PROTOPLASMIC SYSTEM 



all cells nucleo-cytoplasmic organization is visible and 

 ecto-endoplasmic differentiation is expressed. 



Nuclei may vary in size, but their size is generally 

 constant for a given species of cell, though this varies during 

 the process by which the cell reduplicates itself. In eggs 

 it also varies during the end-stages of ripening, as will be 

 shown later. 



The nucleus also shows diversity in form. Nuclei though 

 most frequently spherical or oval or tending toward these 

 two forms, may be quite irregular as those found in white 

 cells of human blood, for example. The amoeboid, irreg- 

 ularly form-changing nuclei in the spinning gland cells of 

 butterflies have often been described. Again, during the 

 process of division an otherwise spherical or oval nucleus 

 may be very irregular due to incomplete or slow fusion of 

 the separate chromosomes into one nucleus. This is 

 clearly shown in nuclei of some eggs, as those of the thread 

 worm, Ascaris, the water-flea, Cyclops. In these the 

 nucleus may be regarded as a composite of many smaller 

 nuclei, each containing one chromosome. Although this 

 condition is not met with in all animal cells, it may 

 be regarded as a fundamental condition of nuclear 

 structure. 



That the chromosomes are the best known structures 

 in the cell is due in part to the fact that they can be so 

 easily studied because of their affinity for colors, which 

 gave them their name. In the living cell^ they often appear 

 as refringent drops of the same shape and approximately 

 the same size as when fixed and artificially stained with one 

 or another dye. A constant number of chromosomes is 

 characteristic for the cells of a given species of animal. 

 This number is best ascertained in fixed and stained cells, 

 though it can be made out in some living cells, while they 



^ Flemming, iSjg^ Peremeschko, 1879. 



49 



