AND INHERITANCE IN PEDIASTRUM. 387 



axes. Taking cell 4 as an example, the pressures upon it are from 

 the cells i, 3, 11, 12, 13 and 5 and in tending to round up while 

 developing its pressure relations with these cells it forms the wedge- 

 shaped surfaces of contact inclined to each other at the character- 

 istic angles of 120°. The axes of major growth of the cells after 

 the early stages are just what they would be expected to be on the 

 principle of functional hypertrophy, though as shown below, the 

 cells are capable of reaching their typical form even when their con- 

 tact with other cells has been reduced to a minimum (Figs. 29-33). 

 The strongly four-lobed cells are in their form adapted to the 

 expression of a typical surface tension configuration for the whole 

 sixteen-celled colony, but this form does not apparently depend for 

 its expression in ontogeny on the contact relations to which it is 

 so perfectly adapted. Whether in phylogeny, as noted, it was not 

 a result of the cellular interactions existing in the colony is, of 

 course, quite another question. The fluctuating variation? in the 

 form of the cells and of the intercellular spaces are the direct ex- 

 pression of the effects of functional response to environmental influ- 

 ences operating in ontogeny. But the cell is able independently to 

 develop its fundamentally four-lobed form. 



It may be that we have here a case of an adaptive form char- 

 acter which arose in direct response to and as it were determined by 

 environmental factors of cellular interaction which has become so 

 fixed that it is now transmitted in cell division and comes to typical 

 expression without the need of the stimuli of adhesion and pressure 

 relations which originally called it into being. The considerations 

 here developed apply, of course, especially to the sixteen-celled 

 colony. The case of the thirty-two-celled colony will be considered, 

 as noted below, in another connection. 



It is during growth that the intercellular spaces are developed. 

 As the cells of the colony reach their mature size the intercellular 

 spaces become again relatively smaller. The cells now increase 

 their volume by expanding in the direction of least resistance, that 

 is, toward the intercellular spaces. As they prepare for reproduc- 

 tion by swarmspore formation this swelling becomes very marked 

 (Fig. 8). The ovoid spaces become triangular, as does the large 



