PROTOZOA 



of its inner surface constantly worn away by the erosion of its 

 discharges. Growth — and that growth " interstitial," operating 

 at every point of the protoplasm, not merely at its surface — is 

 a character of all living beings at some stage, though they may 

 ultimately lose the capacity to grow. Nothing -at all comparable 

 to interstitial growth has been recognised in not-living matter.^ 



Again, when an Amoeba has grown to a certain size, its 

 nucleus divides into two nuclei, and its cytoplasmic body, as we 





Fro. 4. — Amoeba poli/jjuilia in successive stages of equal tission ; nucleus dark, con- 

 tractile vacuole clear. (From Verworn, after F. E. Scliulze.) 



may term it, elongates, narrows in the middle so as to assume 

 the shape of a dumb-bell or finger-biscuit, and the two halves, 

 crawling in opposite directions, separate by the giving way of the 

 connecting waist, forming two new Amoebas, each with its nucleus 

 (Fig. 4). This is a process of " reproduction " ; the special case 

 is one of " equal fission " or " binary division." The original 

 cell is termed the " mother," with respect to the two new ones, 

 and these are of course with respect to it the " daughters," and 



' The growth of crystals is a mere superficial deposit, and cannot at all be 

 identified with xirotoplasniic growtli. 



