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 é 
essential cell substance, the cell may — gy¢, 1—Blood cell of a crab 
contain certain so-called cell prod- (after Hazcxe1). Show- 
ucts, substances produced by the life oe eee 
processes of the protoplasm. The circular spot) and gran- 
cell may thus contain water, oils,  jinsinthecrtopaem 
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 
