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 
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 
