992 S. J. HOLMES 
that of a factory, can be performed on a large or a small scale, 
but as there are many functions to be discharged, and as one cell 
cannot do the work of another, a point naturally has to be reached 
somewhere when a further reduction of the number of cells brings 
operations to a standstill. Matters might work out, however, in 
a different way by effecting a general simplification of structure, 
such as occurs in the reduction of Hydra. This simplification of 
structure, which has been compared to a reversal of embryonic 
development, does not proceed in the planarian very far. The 
loss of sex ducts and associated organs is of doubtful significance, 
since these parts often atrophy at certain periods in adult indi- 
viduals. In attempting to carry reduction below a certain size 
the cut ends of the pieces heal over and there is little further 
change; the organism does not transform itself into a simple 
embryonic stage, and we cannot with safety speak of the reversal 
of developmental processes (if it be really such) beyond perhaps 
a few retrogressive steps. rs 
