THE DEVELOPMENT OP THE EMBRYO ITSELF. 103 



the cells of the mesoblast by outgrowths of nuclear prolifera- 

 tion, and in the case of vessels (Fig. 143) extension of processes, 

 fusion, and excavation. 



Flo. 106.— Biagram of the heart and principal arteries of the chick (Qnain). A repre- 

 sents an earlier and 6 and C later stages. 1, 1, omphalo-meeenteric veins; 2, auri- 

 cle; 3, ventricle; 4, aortic bulb; 5,5, primitive aortse; 6,6, omphalo-mesenteric 

 arteries; A, united aortee. 



The fore-gut is formed by the union of the folds of the 

 splanchnopleure from before backward, and the hind-gut in a 

 simUar manner by fusion from behind forward. 



Pig. 107.— Biagrammatic outlines of the early arterial system of the mammalian em- 

 bryo (after Allen Thomson). A. At a period corresponding to the thirty-sixth or 

 thirty-eighth hour of incubation. B. Later stage, with two pairs of aortic arches 

 h, bulbns arterioBus of heart; v, vitelline arteries; 1—5, the aortic arches. The 

 dotted lines indicate the position of the future arches. 



