GENERAL RETROSPECT. 163 
The formative process of the cells of plants was clearly 
explained by the researches of Schleiden, and appeared to be 
the same in all vegetable cells. So that when plants were 
regarded as something special, as quite distinct from the 
animal kingdom, one universal principle of development was 
observed in all the elementary particles of the vegetable or- 
ganism, and physiological deductions might be drawn from it 
with regard to the independent vitality of the individual cells 
of plants, &e. But when the elementary particles of animals 
and plants were considered from a common point, the vege- 
table cells seemed to be merely a separate species, co-ordinate 
with the different species of animal cells, just as the entire 
class of cells was cv-ordinate with the fibres, &c., and the 
uniform principle of development in vegetable cells might be 
explained by the slight physiological difference of their elemen- 
tary particles. 
The object, then, of the present investigation was to show, 
that the mode in which the molecules composing the elemen- 
tary particles of organisms are combined does not vary 
according to the physiological signification of those particles, 
but that they are everywhere arranged according to the same 
laws ; so that whether a muscular fibre, a nerve-tube, an ovum, 
or a blood-corpuscle is to be formed, a corpuscle of a certain 
form, subject only to some modifications, a cell-nucleus, is uni- 
versally generated in the first mstance; around this corpuscle 
a cell is developed, and it is the changes which one or more 
of these cells undergo that determine the subsequent forms of 
the elementary particles ; in short, that there is one common 
principle of development for all the elementary particles of 
organisms. 
In order to establish this point it was necessary to trace 
the progress of development in two given elementary parts, 
physiologically dissimilar, and to compare them with one 
another. If these not only completely agreed in growth, 
but in their mode of generation also, the principle was 
established that elementary parts, quite distinct im a_phy- 
siological sense, may be developed according to the same laws. 
This was the theme of the first section of this work. The 
course of development of the cells of cartilage and of the 
