244 BIOLOGICAL PHYSICS 



process of, embryonic, as well as, foetal, development. 

 Thus, it appears, when the process of differentiation of 

 the neurenteric canal becomes effected, by posterior 

 division, and anterior union, or by the development of 

 the coccygeo-anal, and pituitary, metamorphic, separating, 

 and uniting, mechanisms, or organisms ; it culminates dur- 

 ing the formation of the vertebral column (Figs. 103, 104); 

 and declines in prominence, as that part of the skeleton 

 reaches the stage of advanced, if not completed, ossifica- 

 tion, in its formation ; and persists, until the vertebral 



mr 



FIG. 103. TRANSVERSE SECTION OF AN EMBRYO RABBIT OF 9 DAYS AND 



2 HOURS IN THE MIDDLE DORSAL REGION. lf& (From Kolliker.) 



mr, medullary tube ; uiv, proto vertebral mass ; h, epiblast ; hp, parietal mesoblast ; 

 dfp) visceral division of the mesoblast; //, pleuro-peritoneal cavity between them ; 

 ung l primitive segmental duct ; g, vessels in the visceral mesoblast ; ch, notochord ; 

 dr, intestinal groove of the hypoblast. 



centres have progressed to the full extent of their peri- 

 pheral boundaries, and have assumed their proper position 

 in the structure of the fully-developed spinal column, when 

 it begins to decline in prominence of anatomical features, 

 and shrinks into the condition of a survival of itself. 

 What, we ask ourselves, does all this suggest ? It 

 suggests that the notochord, at this period of embryonic 

 development, is performing a most important function in 

 the economy of growth and evolution of the embryonic 

 formation skeletal structures, and from its intimate 

 relationship, at its anterior extremity, with the process of 

 differentiation, or rather union, of the already divided 

 extremities of the neurenteric canal, it receives from that 

 canal and its proper, neuro-mural, or structural, elements, 



