PATTEN— COOPERATION AS A FACTOR IN EVOLUTION. 517 



that they are persistently confused in practically all of our text books, 

 a condition chiefly due to the incubus of an ancient terminology that 

 had its origin before the days of embryology ; a terminology based 

 on the position of the animal in locomotion, not on its internal struc- 

 ture and its mode of growth. 



The peculiar advantages of bilateral apical growth over the radial 

 plan, are apparent when we examine the embryo of one of the higher 

 invertebrates, such as a scorpion, or Limulns, where all the impor- 

 tant organs are clearly laid down in triaxial gradients coincident with 

 the chief lines of conveyance (Fig. 3). 



Fig. 3. Diagrams (mercator projections) of arachnid embryos show- 

 ing how apico-bilateral growth on a spherical surface follows the lines of 

 easiest conveyance and least resistance, thereby determining the chief mor- 

 phological features of the embryo. From Patten, " The Evolution of the 

 Vertebrates and their Kin." 



In the radial plan, there are only two unlike sides, oral and 

 aboral, while innumerable homologous points on corresponding radii, 



