CELL THEORIES. 127 
there need be little doubt that pus-corpuscles are 
derived from white blood-corpuscles, there also 
need be no more doubt that they likewise origin- 
ate both fissiparously and endogenously in the 
tissues. 
I have used the expression “common or undiffer- 
entiated corpuscles;” and on this subject it may 
be necessary to make some remark. The vital 
powers may be enumerated as irritability or sensi- 
bility, contractility, nutrition or elaboration of sub- 
stance, and reproduction. All these properties are 
possessed by an amceba and by ameeboid cor- 
puscles. But every separate living organism does 
not possess them all; there are individuals with- 
out reproductive power. In like manner, all the 
vital units do not possess throughout life all vital 
properties; but zz the process of differentiation 
one property becomes exalted, while another is lost. 
Muscular fibres, nerve-corpuscles, the corpuscular 
elements of peripheral nerve-terminations, and 
secreting corpuscles illustrate this, All of them 
have lost the reproductive power; muscular fibres 
have exalted contractility, nerve-terminations ex- 
alted sensibility, and secreting corpuscles a highly 
developed elaborating power. [In brain-corpuscles 
the proliferating power may reappear in patho- 
logical circumstances, but it does so at the expense 
