A CRITICISM OF THE CELL-THEORY. 148 
in Unio, there result from the division of the ovum separate 
protoplasmic corpuscles, as distinct from one another as one 
room in a house is distinct from another, each of which is not 
only separate, but contains within itself definite, and probably 
limited, qualities (at least at stages beyond eight or sixteen 
cells), One might almost say that, after the earliest stages, 
each blastomere has a definite task allotted to it, which it 
faithfully and punctually performs, according to a prescribed 
course. To each, it might be said in figurative language, is 
given material, which it must place, not anywhere, but in one 
particular part of the edifice. 
In considering these very remarkable researches, it is not 
sufficient, for the present purpose, to say that no connection 
between the blastomeres was observed. Such connections 
may have existed and have been overlooked; as the con- 
nections, which undoubtedly exist, between plant cells were 
for a long time overlooked. But, a priori, such connections 
are improbable. For, as has been said, the qualities of each 
blastomere are limited. ach is specialised before any form 
changes become visible ; each plays one part, and one part 
only in tissue formation. If their protoplasm were continuous, 
being made so by uniting strands, then, as Mr. Sedgwick has 
expressed it, the molecular constitution of any part would in 
time spread through the whole mass. But the molecular 
constitution of the blastomeres must be different, for their 
manifestations are different, and we may possibly see, in this 
case, some explanation, obscure though it may be, of the iso- 
lation of the form elements from one another. 
Further than this, there is objective proof that the cells 
constituting the early embryos of these forms are separate. 
They exhibit remarkable shiftings of position, which render 
the existence of connecting strands of protoplasm highly im- 
probable, and the migrations of some cells—e.g. those in 
Nereis named c+: and d !*>* by Wilson—are of such an extent 
that, if there were protoplasmic continuity, they would be 
impossible. 
It is no exaggeration to say that this is evidence which 
