THE EVOLUTION OF CELL TYPES AND CON- 
TACT AND PRESSURE RESPONSES 
IN PEDIASTRUM . 
By R. A. HARPER 
Columbia University 
I desire to present at this time an outline of the evolutionary 
development in the genus Pediastrum, noting especially also the 
relation of the forms of the various cell types which characterize 
the species to the intercellular biogenetic reactions through which 
the colonies get their characteristic configurations. We shall find 
in the subgenera a series of well-marked groups, in each of which 
the particular initial primitive cell form foreshadows all the types 
which have appeared in the evolution of this particular series. 
The differences which characterize the species in many cases 
pass over into each other by very finely graded variations, so 
that there has been the greatest possible confusion and uncer- 
tainty among systematists as to whether certain types should be 
considered species, varieties, or mere form races. An orthogenetic 
trend of development can be recognized in the Diactinia and 
Tetractinia especially and the forms illustrate such series of con- 
tinuous variants as Jennings ('16) has produced by selection in 
Difflugia corona. 
The genus, as a whole, on the other hand, presents a series of 
groups, the subgenera, which just as plainly differ by discontinuous 
characters. In most cases it is necessary to assume a return to 
the primitive undifferentiated cell type of the simplest species 
in order to conceive the method of origin of the subgenera. 
Ап orthogenetic trend for the whole genus can perhaps be 
recognized in the simple tendency to develop spinous projections 
on the body of the cell, but we need to know more of the relation 
of such changes of form to the ultimate constitution of the cell 
before we can be sure that there is any common background for 
the tendency which has led to the formation of the one-spined, 
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