THE REGION OF THE SPIXAL CORD 



413 



-Mes 



-My 

 -Sd 



-Met 



VMes 



c 



Fig. 160. — A, Diagram 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 Folds ; C, Diagram to illustrate the Completion of the Verte- 

 brate Type by the Meeting of the Pleural Folds in the Mid-ventral 

 Line and the Obliteration of the Atrial Cavity. 



.1/., alimentary canal; N., nervous system; My., myotome; PL, pleuron ; App., 

 appendage; Nepli., nephrocoele ; 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. 



