424 HARPER— ORGANIZATION, REPRODUCTION 



The peripheral lobes are basally placed and one of them functions 

 for attachment and yet they retain their more tapering form. 



It is over and over illustrated in abnormal colonies that the 

 polarity shown in the difference between the basal and peripheral 

 lobes is a matter of cell organization and not of colony organization. 

 A very characteristic case is shown in Fig. 34. Here in an other- 

 wise quite regular colony one of the ten peripheral cells has been 

 reversed in position and thrust partly back into the second series. 

 The one free peripheral lobe is quite normally developed though 

 pointing toward the central cell of the colony. The diagonally oppo- 

 site basal lobe which is free to grow radially outward has shown no 

 tendency to do so. The second true peripheral lobe has had its 

 natural growth tendencies quite inhibited by the limitations of space 

 in which it finds itself. The tendency to functional hypertrophy if 

 operative here is not equal to the production of a normal peripheral 

 lobe under such conditions. The whole grouping in such a case 

 gives a very clear picture of the exact part played by heredity and 

 environment respectively in morphogenetic processes. 



Such examples as are shown in Figs. 25-30 can be multiplied 

 almost without limit and it is clear that however much the contact 

 and pressure relations in the group may have influenced the evolu- 

 tionary processes by which such oval cells as those of Gonium be- 

 come the four-lobed oblong cells of Pediastrum, at present these 

 cells are able to attain their characteristic forms, diagnostic for the 

 species, when almost entirely free from their normal environmental 

 relations with the other cells of the colony, 



Nitardy figures several marked cases ('14, Taf. VI., p. 2) in 

 which the single spinous outgrowth and general triangular form of 

 the cells of P. simplex are shown to be an hereditary growth-habit 

 of the cells rather than a response to their pressure relations of 

 orientation in the colony. In the figure referred to a peripheral 

 cell is shown with its poles reversed and the spine projecting toward 

 the center of the colony and an intercellular space quite as in my 

 Fig. 30 described above. 



It would be natural, perhaps, I0 expect that the four-lobed form 

 should be strictly epigenetic and achieved anew by each generation 



