PLATE 6 



EXPLANATION OF FIGURES 



27 Section through the heart of the same meroph\st showing completion of 

 ventral 'body wall' and remains of ventral mesentery of the heart. Note the 

 well developed endocardiimi and dorsal aortae. 



28 Section through the fore-gut of meroplast Type II, no. 52, showing numer- 

 ous endothelial cysts and mesothelial folds. The entire body wall is covered 

 with ectoderm. Section is slightly frontal. 



29 Ti-ansverse section through the multiple unilateral heart anlage of mero- 

 plast Type I, no. 111. 



30 Transverse section through the posterior axial region of meroplast. Type 

 II, no. 124, showing unequally developed dorsal aortae. 



ABBREVIATIONS 



Aort., aorta 

 Ed., ectoderm 

 Endc, endocardium 

 Endth., endothelium 

 Ent., entoderm 

 Myc, myocardium 



Pai'. mes., parietal mesoderm 

 Ph., pharynx 



Pv. mch., prevascular mesenchyme 

 Vis. mes., visceral mesoderm 

 X, point of fusion of ectoderm and 

 entoderm 



142 



