384 HARPER— ORGANIZATION, REPRODUCTION 



smaller spaces also form two pairs, /, p and h, I, placed as are the 

 larger pairs but with the nearer pair toward the n pole of the colony. 



In their relation to the cells between which they occur the inter- 

 cellular spaces form two classes, those formed at the points where 

 three cells come together and where three walls intersect and those 

 formed by retraction of the plane contact walls of two adjacent 

 cells ; in other words, the intercellular spaces which are bounded by 

 three cells and those which are bounded by two cells. There are 

 six of the first class, five in the outer series, and one, a, in the inner 

 series, and there are eight of the second class, five in the outer series 

 and three in the inner series. 



These spaces, as noted, get their outlines and positions from the 

 tendencies of the cells to assume their hereditary four-lobed form 

 and not merely as an expression of surface tension and rounding 

 up such as results in the triangular intercellular spaces in many loose 

 parenchymatous tissues and in the colony of Gonium. The final 

 and definite form of each cell in which it differs from its neighbors 

 arises during growth and in irregular colonies these differences may 

 be very marked (Figs. 25-27). They may consist in inequality in 

 the length of the lobes with greater or less blunting of their tips, 

 curving of the lobes, or deformation of the whole trapezoidal out- 

 line of the cell till it becomes rhomboidal or some other form. The 

 growth of the cell will result in the protoplasm as flowage and ten- 

 sions in the direction of the growing lobes and such tensions exerted 

 at four points of the viscous mass will tend to produce a simple cate- 

 noidal deformation of the whole such as arises in all semi-fluid, 

 viscous bodies under tension of any sort. Retraction would nat- 

 urally occur on the surface of contact midway in the regions of ten- 

 sion simulating a tendency of the whole mass to break up into four 

 droplets. The curves bounding the intercellular spaces in the fig- 

 ures (see Fig. 5 especially) are the obvious expression of tensions 

 exerted on the cell mass in the direction of the four lobes. The 

 degree to which this tendency to catenoidal deformation will come 

 to expression will depend on the viscosity of the protoplasm, the 

 adhesion of the cells to each other, etc. It is in these particulars 

 that the species differ and the same species may under varying con- 

 ditions or at different stages of development differ in such charac- 



