EMBRYOLOGY 



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of mesoblast and the ventral hypoblast of the gut, and the 

 muscular tissue is formed by a folding of the splanchnic 

 layer itself. The space between the splanchnopleure and 

 somatopleure in the region of the heart, which at this time 

 is continuous with the rest of the ccelom, forms the peri- 

 cardial cavity. The communication between the pericardial 



Fig. 379. — A, B, and C, transverse sections through the ventral wall 

 of the throat of frog embryos of different ages, showing successive 

 stages in the development of the heart. — -From Bourne. 



ep., Epiblast ; ky., hypoblast ; mes. t mesoblast ; eth., endothelial lining of heart ; 

 ht., heart ; pc, pericardial cavity ; $., sucker ; so., somatic layer of mesoblast ; 

 spl. , splanchnic layer of mesoblast. 



and abdominal cavities is abolished by the formation of the 

 great veins. The venous system is at first arranged on the 

 same plan as in the dogfish, with two ductus Cuvieri and 

 anterior and posterior cardinal veins. Subsequently the 

 posterior cardinal veins disappear and are replaced by 

 the inferior vena cava, the ductus Cuvieri becoming 

 the superior venae cavse. The arterial system of the 

 tadpole closely resembles that of a fish, The conus 



