382 HARPER— ORGANIZATION, REPRODUCTION 



II., is just as conspicuous here as in the case of P. Boryanmn. The 

 delicacy of the pressure and contact responses which prevents the 

 placing of six, the normal least-surface number, around the central 

 cell, thus leaving only nine for the peripheral series, and resulting 

 in quite asymmetric contact relations between series II. and III., 

 affords good evidence of the efficiency of cellular interactions in the 

 production of morphogenetic adjustments. 



The four-lobed form of the cells and the spacing of the five cells 

 of series II. about the central cell and the ten cells of series III. 

 about the five cells of series II., leave free intercellular areas and 

 the symmetrical distribution of these areas in such fashion as to 

 maintain most perfectly the rigidity of the colony and the equality 

 of the cells has led to the development of one of the very obvious 

 differences between P. asperum and P. Boryanum. This is per- 

 haps the most conspicuous difference between the two species. The 

 cells instead of being in contact on the entire extent of their adjacent 

 surfaces show a series of intercellular openings which perforate 

 the plate-shaped colony like the holes in a sieve. An intercellular 

 space is formed between each pair of contact walls of the sixteen 

 cells. They are of such form and are so placed that the interior 

 cells of the colony have the four-lobed form of the peripheral cells. 

 The cells of the whole colony are thus, as noted above, much less 

 differentiated in form than those of the colony of P. Boryanum. 

 The inner cells differ from the outer cells, broadly speaking, only 

 in that their peripheral lobes are blunted and shortened where they 

 meet the inner lobes of the outer series. P. asperum conforms 

 much more nearly to the definition of a coenobe as a coldny of cells 

 quite similar in form and function. 



One might conclude that we have here a simpler type in which 

 cell differentiation has not yet gone so far as in P. Boryanum. I 

 am inclined to the view, however, that in reality P. asperum is the 

 more specialized type and expresses more fully the form-determin- 

 ing tendencies which have led to the development of the spines or 

 lobes on the peripheral cells of the colony of P. Boryanum. 



As we shall find from a study of the method of reproduction 

 in the colony, the cells all inherit alike the tendency to the four- 

 lobed form. This is obvious even in the interior cells of the colony 



