A CRITICISM OF THE CELL-THEORY. 163 
subsists in its parts and in the harmonious interaction of those 
parts. 
The falsity of this summary is evident when we consider 
the known facts of vegetable organisation ; the development 
of Peripatus; the union, by means of protoplasmic processes, 
of epithelial, muscular, and connective-tissue cells; the evidence 
lately adduced as to the continuity of the mesoblast in Elasmo- 
branchs, Aves and Mammalia, and other well-known instances. 
The absolute contrary, as expressed by Mr. Sedgwick, is 
equally false, viz. that the metazoon is a continuous mass of 
nucleated vascular protoplasm, subsisting in the unity of its 
mass. For, as I have shown in the earlier part of this essay, 
there are unequivocal instances of distinct isolated cells 
occurring in the embryos of many Metazoa (Nereis, Unio, 
Umbrella, Leptoplana). Moreover Iam convinced, by my own 
studies on the histology of Ceelenterates, that, whilst there is 
organic connection between many of the tissue-cells composing 
these organisms, as was demonstrated long ago by the brothers 
Hertwig, there are many other cells of which such continuity 
cannot be affirmed. 
To deal clearly with the cell-theory, or rather with the inde- 
pendent-life-unit theory which has grown out of it, we must 
split it up into as many separate propositions as it contains. 
These are: 
The multicellular organism is an aggregate of elementary 
parts, viz. cells. 
The elementary parts are independent life units. 
The harmonious interaction of the independent life units 
constitutes the organism. 
Therefore the multicellular organism is a colony (cell- 
republic according to Hickel). 
It is not necessary to follow the theory further into the 
cousequences which are deducible from these propositions, 
e. g. that development consists in the separation of numerous 
individual units from a single primary unit, the ovum. It is 
obvious that the truth of the first proposition in no way 
depends on the truth of those which follow, and that, in fact, 
