THE LIFE OF THE SIMPLEST ANIMALS 3 



acter. The primitive animal cell (Fig. 1) consists of a 

 small mass of a viscid, nearly colorless, substance called 

 protoplasm. This protoplasm is differentiated to form two 

 parts or regions of the cell, an inner denser mass called the 

 nucleus, and an outer, clearer, inclosing mass called the 

 cytoplasm. There may be more than 

 one nucleus in a cell. Sometimes 

 the cell is inclosed by a cell wall 

 which may be simply a tougher outer 

 layer of the cytoplasm, or may be a 

 thin membrane secreted by the pro- 

 toplasm. In addition to the proto-' 

 plasm, which is the fundamental and 

 essential cell substance, the cell may Flo . i._ B iood ceil of a crab 

 contain certain so-called cell prod- < after HAKCKEL). show- 

 ncta, substances produced by the life 'S^ST^^ 



processes Of the protoplasm. The circular spot) and gran- 



cell may thus contain water, oils, 

 resin, starch grains, pigment gran- 

 ules, or other substances. These substances are held in 

 the protoplasm as liquid drops or solid particles. 



The protoplasm itself of the cell shows an obvious 

 division into parts, so that certain parts of it, especially 

 parts in the nucleus, have received names. The nucleus 

 usually has a thin protoplasmic membrane surrounding it, 

 which is called the nuclear membrane. There appear to be 

 fine threads or rods in the nucleus which are evidently 

 different from the rest of the nuclear protoplasm. These 

 rods are called chromosomes. The cell is, indeed, not so 

 simple as the words " structural unit " might imply, but 

 science has not yet so well analyzed its parts as to warrant 

 the transfer of the name structural unit to any single part 

 of the cell that is, to any lesser or simpler part of the 

 animal body than the cell as a whole. 



The protoplasm, which is the essential substance of the 

 cell and hence of the whole animal body, is a substance 



