THE REGION OF THE SPINAL CORD 413 
Fic. 160.—A, Dracram oF SECTION THROUGH A TRILOBITE-LIKE ANIMAL; B, 
DIAGRAM TO ILLUSTRATE SUGGESTED OBLITERATION OF APPENDAGES AND THE 
FORMATION OF AN ATRIAL CAVITY BY THE VENTRAL EXTENSION OF THE 
PLEURAL Foups; C, DiaGRAM TO ILLUSTRATE THE COMPLETION OF THE VERTE- 
BRATE TYPE BY THE MEETING OF THE PLEURAL FoLps IN THE MID-VENTRAL 
LINE AND THE OBLITERATION OF THE ATRIAL CAVITY. 
Al., alimentary canal; N., nervous system; My., myotome; Pi., pleuron; App., 
appendage; Neph., nephroceele; Met., metaccele; S.d., segmental duct; At., 
atrial chamber; V.Mes., ventral mesentery; Mes., mesonephros. The dotted 
line represents the splanchnopleuric mesoblast in all figures. 
