PATTEN— COOPERATION AS A FACTOR IN EVOLUTION. 515 



But none of the higher animals begins its life as an isolated point 

 within a nutrient solution. It is true that an adult animal may be 

 sessile or free, and the mode of life adopted has an important mould- 

 ing influence on its form and action. But what is more important, 

 because more constant and universal, is the fact that practically every 

 individual metazoan, from the very outset, begins its life as a sessile 

 organism, for it is attached to a more or less inert spherical body (the 

 egg yolk), whose store of non-living contents tends to increase in 

 volume with the progress of evolution. The growth of the embryo 

 is initiated at some point in that body, usually near its surface. 

 Under these conditions, the products of growth inevitably tend to 

 take on the form of a four-layered film, growing in an apico-bilateral 

 direction, with a distributing space, or coelom, between the layers, 

 because that way provides the most economic solution of the initial 

 problems of exchange upon which growth depends, and the most 

 accessible places for the accumulation of the products of growth ; 

 moreover it is the only way in which the more voluminous specialized 

 growth of the higher organisms can take place. 



The volume of the egg, or its circumference, at once determines 

 the distance the film has to grow in order to enclose the yolk, and 

 the volume determines the amount of growth that may occur before 

 taking in new food supplies from without. But the volume of the 

 egg food cannot change the graded sequence of time and space con- 

 ditions that must inevitably appear in a film growing over the surface 

 of a nutritive sphere. 



These graded series of time and space relations determine the 

 basic lines of conveyance and growth, which in turn are expressed 

 in terms of structural gradients, or axes and surfaces, such as the 

 gradient to the right and gradient to the left, gradient from head end 

 to tail end, from the neural surface to the haemal surface, and from 

 the outer layers to the inner layers. 



The fact that in all apico-bilateral growth the lines of unlike 

 time and space conditions created by the progress of growth coincide 

 with unlike morphological structures is presumptive evidence that 

 these conditions are essential factors in the creation of those struc- 

 tures and that they are the underlying cause of the homologies in 



