THE NOTOCHORD AND ALIMENTARY CANAL 453 



N. N. Nc. 



N. Nc. 



Fig. 167. — A, Diagram of Section through a Trilobite-like Animal; B, 

 Diagram to illustrate the Suggested Formation of the Notochord 

 from a Ventral Groove ; C, Diagram to illustrate the Suggested 

 Formation of the Post-Branchial Gut by the continuation of the same 

 process of Ventral Groove-Formation, combined with Obliteration of 

 Appendages and Growth of Pleural Folds ; D, Diagram to illustrate 

 the Completion of the Vertebrate Type by the Meeting of the Pleural 

 Folds in the Mid-Ventral Line with the Obliteration of the Atrial 

 Cavity and the Conversion of the Ventral Groove into the closed 

 Alimentary Canal. 



.4/., alimentary cannl; N., nervous system; My., myotome; PL, pleuron ; App., 

 appendage; Kcph., nephrocele; Met., metaccele ; Sd., segmental duct; Mcs., 

 mesonephros ; At., atrial chamber; Nc, notcehord ; H., heart; F., fat body; 

 Ng., notochordal groove. (These diagrams are intended to complete the 

 diagrams on p. 413, which, as stated there, were purposely left incomplete.) 



