i86 



EMBRYOLOGICAL TYPES 



veins, which run from the yolk-cells and the rudiment of the 

 liver. 



The dorsal aorta arises as a pair of longitudinal vessels, 



Fig. 84. — Transverse sections through embryos of Rana, showing the origin 



of the heart. 

 Only the ventral portion of the body is shown. A, early stage ; the cells 

 which will give rise to the endothelial lining of the heart (e) are still scattered ; 

 they lie between the endodermal floor of the gut (ef) and the mesoderm (m) ; 

 the mesoderm contains right and left ccelomic cavities (p) still separated 

 by a septum ; ec, ectoderm. B, later stage, the endothelial cells are begin- 

 ning to arrange themselves, and the ccelomic epithelium underlying them 

 becomes thickened and depressed. C, the endothelial lining of the heart 

 is now a closed tube, and the ccelomic epithelium has folded round it forming 

 its muscular wall (mzv) ; it remains connected with the ordinary ccelomic 

 epithelium above by the dorsal mesocardium (dtti) ; beneath the heart, the 

 septum between the right and left ccelomic cavities has disappeared so that 

 the ccelom is continuous and is now known as the pericardium (p, pc). 



close beneath the notochord. The two remain separate 

 anteriorly, as the lateral dorsal aortas and their prolongations 



