THE ENDOCRANIUM; SUMMARY. 



321 



plate, the longitudinal growth of the trabeculae, the overarching growth of the 

 occipitals, and the longitudinal vertical growth of the haemal plates or palato- 

 pterygo-quadrate arcade. (Compare Fig. 220.) 



9. The endocranium has developed along these lines primarily in response to 

 muscular strain, and its principal axes of growth, therefore, coincide with the 

 axes of strain. But after it has become established phylogenetically, as in the 

 vertebrates, it acquires a new moment of growth that is independent of the 

 direction or location of the muscular strains brought to bear upon it. 



ac. 



P- 



FIG. 220. Diagrams illustrating three stages in the evolution of the arachnid endocranium. A, Phyllopod 

 stage, seen from the neural surface, and in transverse section. The endocranium consists of a flattened basilar 

 plate, located in the region of the oral appendages (mandibles and maxillse) and composed of a thickened trans- 

 verse bar, and two longitudinal ones. B, Primitive arachnid stage; the endocranium extends over the whole 

 of the thoracic region, and shows the beginning of the anterior and posterior cornua, a.c. and p.c., lateral and 

 haemal processes, l.p. and h.p., and marginal ridges, m.r. C, Arachnid stage, showing the fully developed arachnid 

 endocranium, and the principal axes of growth. 



10. The vertebrate embryo picks up the growth of the endocranium at about the 

 highest stage reached in the arachnids. If the growth of the arachnid endocranium 

 advanced still farther along the lines it has already established, it would follow 

 very nearly the later embryonic development of the vertebrate cranium. For 

 example, a. the upward growth of the lateral walls of the trabeculae; b. the forward 

 growth of the supraoccipital ; c. the forward growth of the trabeculae and the 



