THEORY OF THE DEVELOPMENT OF ORGANISMS 107 



alter, the organic material, by its power of reaction, 

 changes its shape in a corresponding fashion. 



As the nature of architectural plans depends 

 upon the properties of the wood, stone, or iron, as 

 they must correspond with the material to be em- 

 ployed (i.e., the span of a roof, the construction of 

 a bridge depend upon the material in shape and 

 weight), so the nature of the organic material 

 determines to a large extent the shapes assumed in 

 the course of growth. 



Shape in many respects appears to be a function 

 of growth in an organic material. 



A few examples will make clear this important 

 relation. A limit is set to increase in the size of a 

 blastosphere by the nature of the material of its 

 walls. Its wall is a membrane, composed of one 

 or more layers of cells ; that this may preserve its 

 curvature, a definite pressure from within must be 

 maintained, proportioned to the cohesive force of 

 the cells ; at the same time the wall of the sphere 

 must be able to withstand the strain and pressure 

 put upon it by external forces. All these, and 

 many other factors less easy to conceive, must be 

 delicately adjusted to one another. If in any 

 direction a definite limit be exceeded, then either 

 the structure will be destroyed by disintegration 

 of the component parts, or a new shape will be 

 assumed. The latter is the event in the case of a 

 living substance capable of reaction. The blasto- 

 sphere, growing beyond its limits, folds into a cup- 

 shaped organism. Did we know all the influences 

 affecting the wall of the blastosphere, then we 



