AND INHERITANCE IN PEDIASTRUM. 421 



the series of seven colonies of P. Borymmm are given in Table 

 VIII., and agree well with those obtained for the selected colony 

 ('i6, p. 98). I have not included measurements of the basal walls 

 of cells 8, 10, 12, 14 and 16, as their tendency to equality is suffi- 

 ciently obvious. 



The corresponding radial walls between the cells of series II. 

 tend to be equal as do also the radial walls between the cells of 

 series III. The indication from the measurements of the selected 

 colony that the walls i and i^ of the central cell should be regarded 

 as typically a little longer than the walls i^ and v' is not confirmed 

 by the measurement of the series, though there is a difference of 

 .2 mm. in the average. Measurements of the dimensions of the 

 central cell in a larger series of colonies would be of interest. Its 

 inherited oblong and two-lobed form, and the pentagonal outline in 

 the colony formed by the bases of the five cells of series II. afford 

 an interesting case of disharmony in morphogenetic factors. 



Inheritance of Cell Form. 



I have pointed out that the four-lobed form of the cells appears 

 immediately in the young colonies and have referred to it through- 

 out as inherited rather than as the direct and epigenetic expression 

 of the pressure and other interrelations of the cells in the growing 

 colony. I have pointed out the adaptation of this four-lobed cell 

 form to the exigencies of group formation, when the number of 

 units is strictly limited by the principle of bipartition. The prin- 

 ciple of least surfaces here requires that five cells instead of six are 

 to be placed about one in the center and ten cells in the third series. 

 This arrangement involves just such a tangential elongation of the 

 individual cells as we find has actually occurred and favors the 

 maximum of compactness in the arrangement i -|- 5 + 10. I have 

 suggested the possibility that the environmental complex may have 

 led in successive generations to the development of this four-lobed 

 form and its fixation as an hereditary character of the cells. Evi- 

 dence that the typical form of the cells can be achieved independ- 

 ently of their being in normal contact and pressure reactions in the 

 colony is rather easy to obtain. Many colonies are found in nature 



PROC. AMER. PHIL. SOC, VOL. LVII, CC, AUG. 20, 1918. 



