240 BIOLOGY AND ITS MAKERS 
the cells were not thought of as uniform elements of organic 
architecture, and no theory resulted. It is true that Malpighi 
understood that the cells were separable “ utricles,’”’ and that 
plant tissue was the result of their union, but this was only 
an initial step in the direction of the cell-theory, which, as 
we shall sce later, was founded on the supposed identity in 
development of cells in animals and plants. Fig. 73 shows 
a sketch, made by Malpighi about 1670, illustrating the micro- 
scopic structure of a plant. This is similar to the many 
drawings of Grew and Leeuwenhoek illustrating the struc- 
ture of plant tissues. 
Wolff.---Nearly a century after the work of Malpighi, we 
find Wolff, in 1759, proposing a theory regarding the organ- 
ization of animals and plants based upon observations of 
their mode of development. He was one of the most acute 
scientific observers of the period, and it is to be noted that his 
conclusions regarding structure were all founded upon what he 
was able to see; while he gives some theoretical conclusions 
of a purely speculative nature, Wolff was careful to keep 
these separate from his observations. The purpose of his 
investigations was to show that there was no pre-formation 
in the embryo; but in getting at the basis of this question, he 
worked out the identity of structure of plants and animals 
as shown by their development. In his famous publication 
on the Theory of Development (Theoria Generationis) he used 
both plants and animals. 
Huxley epitomizes Wolff’s views on the development of 
elementary parts as follows: ‘‘Every organ, he says, is com- 
posed at first of a little mass of clear, viscous, nutritive fluid, 
which possesses no organization of any kind, but is at most 
composed of globules. In this semifluid mass cavities 
(Blaschen, Zellen) are now developed; these, if they remain 
round or polygonal, become the subsequent cells; if they 
elongate, the vessels; and the process is identically the same, 
