434 HARPER— ORGANIZATION, REPRODUCTION 



in pressure would naturally give a slightly wedge-shaped form to 

 the cells. That this factor is not necessary for the development of 

 the wedge shape of the cell in the formation of the colony now is 

 shown, as I have indicated ahove, by the fact that the cells will de- 

 velop their characteristic forms when practically free from contact 

 or pressure relations. The basal and peripheral axial differentia- 

 tion seems to appear spontaneously now as an expression of the in- 

 ternal organization of the cell whatever may have been its origin, 

 but a slight progressive difference in density from the basal to the 

 peripheral region of the cell may still be one physical condition for 

 such a polarity. 



The plate-shaped form of the colony as a whole with one layer 

 of cells plainly depends for its achievement on the transverse or 

 right and left contact relations between of the cells. The cells are 

 so constituted that they achieve contacts with as nearly as possible 

 equal pressures on their right and left sides and, furthermore, so 

 placed that the one or two lobes or spines lie in the plane of these 

 contacts and hence in the plane of the colony as a whole. Such re- 

 lations can be represented as transverse polarity by the plus and 

 minus or positive and negative analogy, as I have indicated in the 

 colony shown in figure 5. This schematism breaks down in the case 

 of the central cell and for the radial relations of the cells. It is of 

 course only a purely formal representation of the right- and left- 

 sidedness in the cells. There is no visible evidence in the cell itself 

 for the assumption of any structural right and left differentiation. 

 I have found no proof that any particular cell would not find its 

 polarities equally met when rotated through 180° on its radial axis. 

 The only conspicuous evidence of the location of a transverse or 

 tangential axis is in the fact that the spines regularly lie in the 

 plane of the colony. 



The assumption of an axis of polarity vertical to the plane of the 

 colony would seem to meet the requirements of the case equally well. 

 The poles would be such in this case as to prevent the cell coming 

 to rest when they were in contact with other cells. However these 

 polar differentiations are conceived it is obvious that some such con- 

 ditions are necessary for the achievement by free-swimming swarm- 

 spores of a plate-shaped colony of a single layer of cells. That 



