Embryology. 107 
indicate the fact that when cell-growth proceeds 
beyond a certain point cell-division ensues. The 
size to which cells may grow before they thus divide 
is very variable in different kinds of cells; for while 
some may normally attain a length of ten or twelve 
inches, others divide before they measure 7,4, of an 
inch. This, however, is a matter of detail, and does 
not affect the general physiological principles on 
which we are at present engaged. 
Fic. 27.—Fission of a Protozodn. In the left-hand drawing the process 
is represented as having advanced sufficiently far to have caused a 
division and segregation both of the nucleus and the vesicle. In the 
right-hand drawing the process is represented as complete. 2, N, 
severed nucleus; we, severed vesicle; ps, pseudopodia ; f, ingested food. 
Now, as we have seen, a Protozoén is a single cell; 
for even although in some of the higher forms of 
protozoal life a colony of cells may be bound together 
in organic connexion, each of these cells is in itself an 
“ individual,” capable of self-nourishment, reproduction, 
and, generally, of independent existence. Conse- 
quently, when the growth of a Protozodn ends in a 
division of its substance, the two parts wander away 
from each other as separate organisms. (Fig. 27.) 
