164 GENERAL RETROSPECT. 



cells of the chorda dorsalis was compared with that of vege- 

 table cells. Were the cells of plants developed merely as 

 infinitely minute vesicles which progressively expand, were 

 the circumstances of their development less characteristic 

 than those pointed out by Schleiden, a comparison, in the 

 sense here required, would scarcely have been possible. We 

 endeavoured to prove in the first section that the complicated 

 process of development in the cells of plants recurs in those 

 of cartilage and of the chorda dorsalis. We remarked the 

 similarity in the formation of the cell-nucleus, and of its 

 nucleolus in all its modifications, with the nucleus of vegetable 

 cells, the pre-existence of the cell-nucleus and the development 

 of the cell around it, the similar situation of the nucleus in 

 relation to the cell, the growth of the cells, and the thickening 

 of their wall during growth, the formation of cells within 

 cells, and the transformation of the cell-contents just as in 

 the cells of plants. Here, then, was a complete accordance 

 in every known stage in the progress of development of two 

 elementary parts which are quite distinct, in a physiological 

 sense, and it was established that the principle of develop- 

 ment in two such parts may be the same, and so far as could 

 be ascertained in the cases here compared, it is really the 



same. 



But regarding the subject from this point of view we are 

 compelled to prove the universality of this principle of develop- 

 ment, and such was the object of the second section. For so 

 long as we admit that there are elementary parts which originate 

 according to entirely different laws, and between which and 

 the cells which have just been compared as to the principle of 

 their development there is no connexion, we must presume 

 that there may still be some unknown difference in the laws 



a/ 



of the formation of the parts just compared, even though 

 they agree in many points. But, on the contrary, the greater 

 the number of physiologically different elementary parts, which, 

 so far as can be known, originate in a similar manner, and 

 the greater the difference of these parts in form and physio- 

 logical signification, while they agree in the perceptible phe- 

 nomena of their mode of formation, the more safely may 

 we assume that all elementary parts have one and the same 



