DEVELOPMENT OF THE BLOOD-VESSELS OF THE CHICK. 59 



Fig. n. Section through the same embryo as fig. 10, and passing through the 

 orifice of the omphalo-meseraic vein, of, Omphalo-meseraic vein ; other references 

 as above. 



These two sections shew that the heart is entirely formed from the mesoblast of 

 the splanchnopleure, and that it is formed by the splitting of that part of the meso- 

 blast which has turned to assume its normal direction after being folded in to form 

 the muscular wall of the alimentary canal. In fig. 1 1 the cavities so formed on each 

 side have not yet united, but in fig. 10 they have united. When the folding be- 

 comes more complete the cavities (of, of) in fig. 1 1 will unite, and in this way the 

 origin of the omphalo-meseraic veins will be carried further backwards. In the sec- 

 tion immediately behind section 1 1 the mesoblast had become thickened, but had not 

 split. 



