HARPER: CELL TYPES AND RESPONSES IN PEDIASTRUM 233 
That it is mere analogy is, of course, obvious from the fact that 
the phenomenon is intracellular in the one case while in the other 
it involves the morphogenetic behavior of many-celled tissues and 
organs. Given this change of cell form and the diactinial type 
of colony would result directly, the same polarity and cell inter- 
relations being involved in both cases. 
We have, further, manifestly. orthogenetic groups in most of 
the subgenera. Given the tendency to the development of two 
spines and the species of the two-spined group are at once fore- 
shadowed as are the species of the Monactinia, Triactinia, and 
Tetractinia by the presence of the possibility of developing one- 
spined, three-spined and four-spined or bifid-spined cells, respec- 
tively. Given cells which adhere in groups, at the same time 
having a tendency to develop thick spinous projections with 
catenoidal deformation of the entire cell body, and the whole 
genus is foreshadowed. Such series certainly illustrate one form 
at least of the many types of change which have been characterized 
as orthogenetic, though the use of such a term is not specially 
iluminating in the absence of evidence as to the structural 
features of the cells which have determined their characteristic 
forms. A fuller cytological study of the cells of Pediastrum may 
serve to throw light both on the nature of cell polarities and the 
means by which such orthogenetic transformations are brought 
about. 
The transition from the simplex to the two-spined type, as 
noted, could only come about either by a change giving the new 
character in functional development at once or by a return to the 
primitive integrum type and a new start. The same is true as to the 
possibility of change from the two-spined to the three-spined type. 
On the other hand, the change from the simplex to the trispinous 
form might quite well come about by the gradual development of 
two additional spines with or without the degradation of the 
single existing spine. To be sure, the three spines do not lie in 
the plane of the colony as does the single spine, but the readjust- 
ments which this difference between the two types involves are 
by no means inconceivable. 
Itis notable that in Pediastrum clathratum the interior cells show 
almost as fully developed lobes as those on the periphery and in 
