GENERAL RETROSPECT. 163 



The formative process of the cells of plants was clearly 

 explained by the researches of Schlciden, 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, &c. 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 co-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 instance; 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 elementaiy 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 in 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 



