MEMBEANES OF THE OVUM. 



749 



the heart through the lungs into the left side of the heart, the other 

 from the left side of the heart through the system into the right side of 

 the heart. 



Fig. 375. 



Fig. 376. 



Membranes of the ovum. 



(2135.) We see, therefore, that of the five pairs of vascular branchial 

 arches which at first by their union formed the aorta as in Fishes, those 

 of the first pair on both sides and of the fifth on the left side speedily 

 disappear. The third on each 

 side become the brachio-ce- 

 phalic trunks, the fourth of 

 the right side becomes the 

 descending aorta, while the 

 fifth of the right side and the 

 fourth of the left side are con- 

 verted into the pulmonary 

 arteries. The very short 

 trunk common to the two 

 pulmonary arteries, as well as 

 the equally short trunk of the 

 aorta, properly so called, are 

 produced by the transforma- 

 tion of the single cavity of 

 the original " bulbus arteri- 

 osus" into two distinct canals, 

 and thus this wonderful me- 

 tamorphosis is accomplished. 



(2136.) About the one 

 hundred-and-twentieth hour 

 from the commencement of incubation, the vascular layer of the blasto- 



