150 GILBERT C. BOURNE. 
resides, not in the organism as a whole, but in the separate 
elementary parts, the cells.” 
The definitions of Hertwig are a re-statement in other words 
of the salient features of the theory of Schwann, and it is an 
error to speak of an unsubstantial cell phantom. Nor is there 
any unsubstantiality about the cellular theory of development, 
which, I may remind my readers, originated with Remak, 
The cellular theory of development, taking as its starting point 
the conclusions of Schleiden and Schwann that all organisms 
are cells or composed of an aggregate of cells, states that 
every cell is formed by the division of a pre-existing cell, not 
as Schwann had supposed, by differentiation within a structure- 
less cytoblastema.! Hence Virchow’s well-known aphorism, 
“ omnis cellula e cellula,”’ which, besides denying abiogenesis, 
expresses the cellular theory of development as succintly as 
possible. 
It would have been a great advantage to his own argument, 
and also to his critic, if Mr. Sedgwick had given the clear and 
authoritative expositions of the cellular theory which lay ready 
to hand, instead of confusing the issue by a whimsical account 
of his experience of morphological teaching. 
Let us now examine the cell-theory, as stated by Hertwig, 
in the light of our present knowledge of animal and vegetable 
structure. 
It would not be a difficult task to demonstrate the general 
truth of Virchow’s aphorism. Wherever there is a cell, it may 
be shown to be the product, and generally the immediate 
product, of a pre-existing cell. But it would seem that some 
biologists have added an unwarrantable corollary to Virchow’s 
generalisation, and would say, ‘‘ Nil nisi cellula e cellula,” 
Now from a certain aspect this might be considered true; 
everything depends on the question as to what is a cell? 
Hertwig has pointed out, with much truth, that our pre- 
sent conception of a cell is inseparably connected with our 
conception of protoplasm. We are still very far from under- 
1 Mr, Sedgwick appears to have leanings towards a cytoblastema, as I 
shall show further on, 
