439 
or in the putrefaction of vegetable or animal matters, these may be 
seen to soften, lose their peculiar structure, break up, and ultimately 
be reduced to a molecular condition. 
We shall subsequently see that these two kinds of molecules are 
constantly changing places, or, in other words, molecular matter 
formed from the process of disintegration may, when placed under 
peculiar circumstances, become the basis of matter which undergoes 
development. In nature, the breaking down of one substance is 
the necessary step to the formation of another, and the histolytic or 
disintegrative molecules of one period become the histogenetic or for- 
mative molecules of another. This fact constitutes the basis of the 
law which I shall subsequently seek to establish. 
II. The author pointed out, in the second place, that these mole- 
cules are governed by forces, which induce among them a variety of 
movements, and cause them to combine in definite ways. This 
force, which we may call molecular force, is altogether independent 
of cell, nucleus, or other form of structure. 
1 st, He alluded to the well-known molecular movements described 
by Robert Brown. These vibratile, circular, serpentine, or irregular 
motions may be observed whenever molecules are suspended in fluids 
of certain densities, but are too well known to require notice here. 
They occur altogether independent of organised structures, and must 
be regarded as in their nature purely physical. 
2d, The peculiar movements observed in the interior of cells 
vegetable or animal, and during the putrefaction of organic matter. 
The former are seen in the large vegetable cells of the Chara, Yallis- 
neria, and Tradescantia, among plants ; and those of chyle, the yolk 
of the egg, and of the salivary cell among animals. The author had 
frequently watched the formation of the latter in putrid fluids. A 
scum composed of molecules collects on the surface ; gradually several 
of these unite in minute filaments more or less long, which assume 
vibratile or serpentine movements. They are then called vibriones. 
It has been much disputed whether this class of molecular motions 
be physical or vital. 
3 d, The movements which are unquestionably vital that occur in 
the molecules of the yolk, on the entrance into the ovum of the 
spermatozoid. Here it cannot be maintained that the results are 
purely physical, because in different ova we see such widely varying 
VOL. IV. 3 N 
