90 GENERAL BIOLOGY 



Just outside of the nucleus and within the cytoplasm, there is usu- 

 ally found a tiny circle with a dot in the center. The dot itself is called 

 the centrosome ( ) and the circle about it the at- 



traction sphere, or centrosphere. 



There are little perforations through the nuclear wall so that there 

 is a direct connection between nucleoplasm and cytoplasm. 



CELL INCLUSIONS AND CELL PRODUCTS 



Bodies of a solid nature, not protoplasmic, are common to many 

 cells. These are pigments, oil, fat, crystals, glycogen, starch, chlorophyl, 

 etc., and are commonly spoken of as cell inclusions, though as a matter 

 of fact only foreign substances such as bacteria, etc., should be called 

 inclusions. Starch and chlorophyl are found almost exclusively in plant 

 cells. By these inclusions the shape of the cell is often changed, and 

 particularly the position of the nucleus. Fat gathers at one end of the 

 cell, crowding the nucleus to the opposite extremity and displacing the 

 cytoplasm to the periphery, mostly to that end of the cell occupied by 

 the nucleus. Pigment may be in solution, more frequently in granules, 

 and always is found in the cytoplasm, not in the nucleus. Vacuoles are 

 very common to most cells. These vary in number and size and are 

 usually spherical cavities filled with fluid secreted by the protoplasm. 

 The vacuoles contract, often with considerable regularity, and, as a rule, 

 empty to the surface of the cell. Waste products are in this way elimi- 

 nated from the body of the cell. 



The constituents of a typical cell may then be summarized as fol- 

 lows : 



1. Cytoplasm, the protoplasm that surrounds the nucleus, consist- 

 ing of: 



(a) Spongioplasm, a reticulum or fibrillar network ; 



(b) Hyaloplasm, a fluid portion, also called cytolymph ; 



(c) Cell membrane, often absent in animal cells. 1 



2. Nucleoplasm or karyoplasm, the protoplasm of the nucleus : 



(a) Nuclear membrane, frequently absent ; 



(b) Chromatin, network that stains easily ; 



(c) Limn, closely allied to the chromatin but does not stain ; dis- 

 solves in distilled water ; 



(d) Nuclear sap, a fluid perhaps analogous to the hyaloplasm ; 



(e) Nucleolus, spherical body that stains heavily ; 



(f) Nuclear net knots, or karyosomes, false nuclei that are nodal 

 points formed by interlacing chromatin network; 



(g) Centrosome, a small spherical body often found in the cyto- 



J Regarding the cell membrane, it is well to know that, this is a purely relative 'term, just as a 

 drop of chloroform in water, or a drop of water in chloroform, or a bubble of air in water, can 

 be said to have a cell membrane. These are really surface tension phenomena, where the inter- 

 phases of water-chloroform, etc., have equal resistance to each other. In the "cell membrane" we 

 really have naked protoplasm, tending to round up just as the drop of water does in chloroform. 



