622 PATTEN— COOPERATION AS A FACTOR IN EVOLUTION. 



Moreover the arachnid medullary plate, by a process of embryonic 

 invagination and overgrowth, barely falls short of forming a closed 

 chamber. Neither of these processes could be carried much further 

 without either completely constricting the cesophagus, or shutting up 

 the mouth inside a hollow brain. Such an event would be fatal 



new 



Fig. 5. Diagrammatic sagittal sections of the arachnid and vertebrate 

 type of embryos. For further explanation, see Patten, " The Evolution of 

 the Vertebrates and their Kin." 



