386 HARPER— ORGANIZATION, REPRODUCTION 



the less, in my opinion, may have arisen in evolution from the pres- 

 sure and contact relations of the young cells in the common sixteen- 

 celled colony, regarding them merely as surface tension globules. 

 We may conceive, as is described below, that transitorily the young 

 cells at once flatten upon each other and thus give the figure of 

 P. Boryanum with no intercellular spaces. The spaces then start 

 to form with the first growth of the young colony in such a fashion 

 as to make the resulting four-lobed cells as nearly isodiametric as 

 is possible with the numbers involved and the inherited tendency 

 to adhere together in the most compact figure possible (groups of 

 three). It is quite conceivable, then, for the sixteen-celled colony, 

 that out of the incompatibilities resulting from the laws of biparti- 

 tion and surface tension, both operating in the case of an organism 

 whose cells tend to adhere in colonies, the four-lobed form of the 

 cells has been developed from cell forms such as we find in P. 

 integrum. It is easy to recognize certain physical stimuli which 

 have been present ; first, surface tension tending to keep each cell 

 isodiametric ;second, perhaps during growth, functional hypertrophy 

 as we have observed its action in the case of Hydrodictyon; third, 

 adhesion tending to keep the contact surfaces plane ; fourth, cate- 

 noidal deformation, due to growth tension in the direction of the 

 lobes. Each cell is originally like its fellows — a globular or ovoid bit 

 of jelly. Adhesion during the writhing motions of the final stages 

 of the swarming period leads to its being flattened upon its neigh- 

 bor. This may be favored by low turgor at this period, but we 

 have little evidence on this point. In P. Boryanum the cells, as a 

 rule, adhere permanently over their entire original contact surfaces 

 and thus maintain their primary contact relations. In P. asperum, 

 however, the surfaces of the cells tend to separate. The degree 

 and location of this separation may be determined by the tendency 

 of the cells to catenoidal deformation as they grow rapidly in the 

 axes of greatest resistance as developed by their four-lobed form. 

 The cells of the whole colony, with the exception of the central cell, 

 are longer in their tangential axes than in their radial axes, since 

 five cells fill the space which six should occupy in the second series 

 of a least-surface group and ten fill the spaces of twelve in the outer 

 series. Surface tension tends in each cell to equalize these two 



