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The Theories of Cell Development. 
that they are all hinged upon different conceptions concerning cell 
development ; and secondly, that these different theories may be 
reduced to four classes, namely : — 
(a.) Theories which recognize free cell development. 
(h.) Molecular or globular theories. 
(c.) Theories which locate developmental power outside the 
nucleus. 
( d .) Theories which confine developmental power to the nucleus 
alone. 
No sooner was the cell recognized as the true and universal 
agent of development, than the importance of understanding its 
mode of development was seen ; hence we at once encounter 
theories of cell development, and for many years these followed one 
another in rapid succession. 
(a.) Free Cell Development. — This doctrine has at least one 
positive merit, namely, boldness. Commencing, as I have already 
intimated, with Wolf in 1759, it has encountered various ups and 
downs , until, at the present time, it is really the keystone of the 
doctrine of “ spontaneous generation,” so vigorously defended by 
Dr. H. Charlton Bastian. According to Wolf, “ every organ is 
composed at first of a mass of clear, viscous, nutritive fluid, which 
possesses no organization of any kind, but is at most composed of 
globules.”* Subsequently cells are developed, but they are “ mere 
cavities, and not independent entities ; organization is not affected 
by them, but they are the visible results of the action of the organ- 
izing power inherent in the living mass, or what Wolf calls the 
vis essentialis ” (Huxley). This simply amounts to saying that the 
cell is merely a passive, not an active body ; that it is only matter 
thrown into a convenient form for building material, as clay is 
thrown into the form of bricks for the same purpose, while the real 
builder is the so-called “ vis essentialis ” resident in the “ clear, 
viscous, nutritive fluid.” Twenty years ago it was necessary to 
combat this doctrine, but at the present day it does not even merit 
the compliment of criticism. 
We encounter the next phase of free cell development in con- 
nection with the doctrines of Schleiden. This observer — the first 
to comprehend the fact that the cell is the true and ever-present 
basis of developmental action in vegetable tissues — acknowledged 
two distinct methods of cell growth ; one of which, the so-called 
“ exogenous free cell formation ,” must be regarded as a true creative 
act, while the other (“ endogenous cell formation ”) is a mere con- 
tinuance of the process, or cell multiplication. In the midst of a 
perfectly structureless, clear, transparent fluid (blastema, cytdblas- 
* Huxley, loc. cit. 
