Development of the Fourth Day 249 



The vitelline and allantoic arteries are, of 

 course, purely embryonic structures, and dis- 

 appear at the time of hatching, when the yolk 

 has all been absorbed or taken into the digest- 

 ive tract, and the allantois is cast off and left 

 in the shell. 



From the hinder region of the dorsal aorta 

 are given off arteries that carry the blood from 

 the left side of the heart to the various struct- 

 ures of the abdominal cavity. 



The changes that take place in the venous 

 system are, if anything, rather more compli- 

 cated than those that have just been described 

 in connection with the arteries ; but if each 

 step in the development be understood, there 

 should be no difficulty in understanding the 

 entire process ; and a clearer understanding of 

 the adult circulation can be obtained by the 

 study of its development than, perhaps, in any 

 other way. 



The development of the venous system has 

 been described for the first three days of incu- 

 bation. At the end of the third day, it will be 

 remembered, the blood entered the auricular 

 portion of the heart through the large vein 

 that was called, in its entirety, the meatus 

 venosus. The meatus venosus was largely 



