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 xthjo o^ ^^ 

 inch. This, however, is a matter of detail, and does 

 not affect the general physiological principles on 

 which we are at present engaged. 



Fig. 27. — Fission of a Protozoon. 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. «, N, 

 severed nucleus ; vc, severed vesicle ; ps, pseudopodia ;/, ingested food. 



Now, as we have seen, a Protozoon 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 Protozoon ends in a 

 division of its substance, the two parts wander away 

 from each other as separate organisms, (Fig. 27.) 



