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 composing 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 for/nation of 

 cells. This is the chief resn.lt 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 [ire- 

 sent in the first instance, which lies either around or in the infe- 

 rior of cells already existing ; and cells arc formed In it in ac- 

 cordance witJi certain laws, which cells become developed ill 

 various ways into the elementary parts of organisms. 



The development of the proposition, that there exists one gene- 



