PRIMARY CAUSES OF DIFFERENTIAL GROWTH. 217 



may be regarded as the inevitable expression of conditions successively created 

 by growth, -or by the conditions under which growth takes place. External 

 environment and natural selection have nothing to do with these conditions, 

 neither does "heredity" initiate, or control, or create them; it merely imitates or re- 

 peats them. Hence, as a creator of the foundations of organic structure, we may 

 eliminate natural selection, and external environment, together with heredity and 

 all its ministering tribe of "corpuscles," as we have eliminated the Gods of Love, 

 of War, and of Peace. 



Let us illustrate our meaning more specifically. If cell division takes place 

 in two superficial planes at right angles to each other, a regularly expanding 

 polygon is formed. With its increasing area, the two division planes will tend to 

 fall into radial and tangential planes, and the polygon will become a flat, circular 

 disc, provided the rate of division in these planes coincides with the ratio between 

 the radii and the circumference of expanding circles. But as soon as such a disc 

 is formed, the central cells are placed under different conditions from the marginal 

 ones, as to nutrition, respiration, and tension; and in addition, the latter are 

 advancing into new territory by their own growth, and at the same time they are 

 crowded over it at a constantly increasing rate, by the division of those cells that 

 lie nearer the center than they do. 



Thus a difference between the local rate of cell division, the local rate of 

 cell displacement, and the local rate of histological differentiation is established. 

 As the local augmentation or diminution of growth produced by these conditions 

 has no constant relation to the increasing radii and circumference, it follows that 

 the actual form of the disc will be a resultant of the various sets of forces. The 

 surface layers will not fit the deeper ones, or the central portions fit the periphery. 

 The disc must, therefore, either change its marginal contour, or its surface con- 

 tours, or both. That is, the areas of unequal growth stresses must express them- 

 selves either in a disc with a broken contour (a symmetrical polygon, hexagon?), 

 or in a circular disc with symmetrically placed infoldings, or eruptions of its surface, 

 or both. (Fig. 119, 4.5.) 



If we now consider the vertical increment of our hypothetical group of cells, 

 it is clear that the conditions change more rapidly in a vertical direction, with 

 increasing thickness, than in a superficial horizontal one, with increasing width. 

 There is consequently a greater modification of the rate of growth and of special- 

 ization in a vertical direction than in a horizontal one. The inevitable result is, 

 therefore, not a homogeneous sphere, but a lens-shaped disc composed of unlike 

 concentric strata, i.e., ectoderm, mesoderm and endoderm, each stratum com- 

 posed of zones of unlike organs, concentric with the point of initial growth. 



Thus a plan of an adult radiate, for example, seen from its oral surface in 

 mercator projection, presents a .succession of ring-like zones, intersected by radii, 

 marking the distribution of like parts. The central area represents the -point of 

 origin of the endoderm, or the blastopore, or the opening to the primitive gut. 

 Around this opening the various organs are arranged in concentric circles that are 



