PLATE 7 



EXPLANATION OF FIGURES 



31 Section through the posterior axis, Type II, no. 149, showing a small 

 aortic anlage which has developed in a region devoid of a coelom. 



32 Section through an early visceral mesodermal proliferation of pre-endo- 

 thelial tissue in the aortic line anterior to the plane of figure 31. 



33 Section bietween the planes of figures 31 and 32 showing a ventrally directed 

 proliferation of visceral mesoderm in the aortic line. 



34 Transverse section through the posterior axial region (Type II, no. 53) 

 showing small dorsal aortae. 



35 Transverse section through the heart region of Type I, no. 34, showing 

 endocardium approximated to entoderm. 



36 Transverse section through anterior heart region of same meroplast show- 

 ing endothelium apparently being delaminated from mesoderm. 



ABBREVIATIONS 



Aort., aorta Pv. tnch., prevascular mesenchyme 



Endc, endocardium Vis. mes., visceral mesoderm 



Ent., entoderm x, point of fusion of ectoderm and ento- 



derm 



144 



