376 



HARPER— ORGANIZATION, REPRODUCTION 



processes and to give evidence as to the possible mode of origin and 

 interrelations of different types. 



The shape of the cell is the main basis of group distinctions in 

 the genus. As Braun ('55) first pointed out, the number of cells 

 in the colony has been shown not to be diagnostic of species or 

 larger aggregates. Fig. 21 shows two daughter colonies, one with 

 sixteen and one with thirty-two cells, both of which came from the 

 same thirty-two-celled .mother colony. The shape of the cells of a 

 given species varies in minor details of proportion about a norm for 

 the species. The form of the cells of different species are of more 

 fundamentally diverging types. 



Fig. I. Fig. 2. Fig. 3. 



Fig. I. Pediastrnm integrum. Cells almost without spines or lobes. 

 The colony shows only fourteen cells but it is possible, as is common for this 

 species, that it is partially two layered. X about 225. 



Fig. 2. P. simplex. Cells with one spine. Configuration i -(- 7. 

 X about 150. 



Fig. 3. P. clathratum. Cells with two very long and slender peripheral 

 and two shorter basal lobes, large intercellular spaces. Configuration 5 -]- 11. 

 Colonies bilaterally symmetrical about the axis m, n. X about 350. 



It is a question of first importance morphogenetically whether 

 these cell forms are hereditary or whether they are newly achieved 

 under the stimulus of the intercellular environment in each genera- 

 tion. It is plain that the form is not inherited directly as such since 

 the two-spined cells do not divide in such a way as to prodtice at 

 once two equivalent two-spined daughter cells. 



The main distinctions in the group are between forms whose 

 cells have no spinous processes or very rudimentary ones, and those 

 whose cells have more or less developed spines. These distinctions 



