GENERAL RETROSPECT. 165 
fundamental principle of development. It was, in fact, 
shown that the elementary parts of most tissues, when 
traced backwards from their state of complete development 
to their primary condition are only developments of cells, 
which so far as our observations, still incomplete, extend, 
seemed to be formed in a similar manner to the cells com- 
pared in the first section. As might be expected, according 
to this principle the cells, in their earliest stage, were almost 
always furnished with the characteristic nuclei, in some the 
pre-existence of this nucleus, and the formation of the cell 
around it was proved, and it was then that the cells began to 
undergo the various modifications, from which the diverse forms 
of the elementary parts of animals resulted. Thus the apparent 
difference in the mode of development of muscular fibres and 
blood-corpuscles, the former originating by the arrangement of 
globules in rows, the latter by the formation of a vesicle 
around a globule, was reconciled in the fact that muscular 
fibres are not elementary parts co-ordinate with blood-cor- 
puscles, but that the globules composimg muscular fibres at 
first correspond to the blood-corpuscles, and are like them, 
vesicles or cells, containing the characteristic cell-nucleus, 
which, like the nucleus of the blood-corpuscles, is probably 
formed before the cell. The elementary parts of all tissues 
are formed of cells in an analogous, though very diversified 
manner, so that it may be asserted, that there is one universal 
principle of development for the elementary parts of organisms, 
however different, and that this principle is the formation of 
cells. Thisis the chief result of the foregoing observations. 
The same process of development and transformation of 
cells within a structureless substance is repeated in the for- 
mation of all the organs of an organism, as well as in the 
formation of new organisms ; and the fundamental phenomenon 
attending the exertion of productive power in organic nature 
is accordingly as follows: a structureless substance is pre- 
sent in the first instance, which lies either around or in the inte- 
rior of cells already existing ; and cells are formed wm it in ac- 
cordance with certain laws, which cells become developed in 
various ways into the elementary parts of organisms. 
The development of the proposition, that there exists one gene- 
