DEVELOPMENT 203 



is a complex mixture of proteins, carbohydrates, lipoids, mineral 

 salts, water, etc. We speak of an organ, tissue, embryo, etc., 



as a complex of cells — this is often justified and it is always 

 convenient in exposition. But the cells are always in contact, 

 or are structurally continuous with each other and often the 

 boundaries between them are obscure, while infrequently we 

 cannot speak of cell boundaries at all. When we can speak of 

 typically bounded cells we recognize in them a very complex 

 morphology : this is obvious when we remember that a " cell " 

 may be any kind of tissue element, such as a nerve-ganglion cell, 

 a muscle-cell, etc., or it may be an organism that is perfectly 

 competent to carry out all the functions of a living thing — or it 

 may be an ovum, when it has the potentialities of a complex 

 multicellular, higher animal, but in this case its " morphology " 

 is, in some way, latent in it and not accessible to minute anatomical 

 study. 



The cell-nucleus, or energid is always a constituent of a cell, 

 or of an organic plasmodium without cell boundaries. It is 

 always a distinctly bounded body, typically spherical in form. 

 The boundary is the nuclear membrane and much of the contents 

 of this may be regarded as proteins, carbohydrates, lipoids, 

 phosphatides, etc. Characteristic of the nucleus, however, is the 

 substance called Chromatin that it contains. Chromatin is usually 

 recognized by the way in which it stains with a class of dye-stuffs 

 — this is the origin of the term. Usually the chromatin is dis- 

 persed through the nuclear body as minute granules, or dis- 

 continuous filaments, or as small rounded bodies called nucleoli. 

 Sometimes, as when the cell is going to divide, the chromatin 

 assumes a definite morphology which is characteristic of the kind 

 of cell. It then becomes segregated into a complex " skein," 

 or a system of filaments, and typically it becomes broken up into 

 a number of bodies called chromosomes. These are (typically) 

 short rods, but they are also described as granules, of many 

 diff"erent forms. There may be from two to nearly 200 chromo- 

 somes in a cell which is in process of division. Typically the 

 number of chromosomes in such a cell is constant for the species 

 of organism that is the cell, or which is a body composed of the 

 cells in question (but the number is often not really constant.) 



The chromosomes are minute bodies that are not far from the 

 limits of visibility, as seen under a high-power microscope. 



