DEVELOPMENT. 39 



establish a most intimate relation between nucleus 



and cytoplasm. 



Cell Inclusions. Bodies of a solid nature, not pro- 

 toplasmic, are common to many cells. These are 

 pigments, oil, fat, crystals, glycogen, starch, chloro- 

 phyl, etc., and are spoken of as cell inclusions. The 

 last two 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 nu- 

 cleus 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 in the cytoplasm, not in the nucleus. Vacu- 

 oles 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 eliminated from the body 

 of the cell. 



The constituents of a typical cell may then be 

 summarized as follows : 



1. Cytoplasm, the protoplasm that surrounds the 

 nucleus, consisting of, 



(a) Spongioplasm, a reticulum or fibrillar network ; 

 (6) Hyaloplasm, a fluid portion, also called cy- 

 tolymph ; 



(c] Cell membrane, often absent in animal cells. 



2. Nucleo plasm or karyoplasm, the protoplasm of 

 the nucleus, 



