178 



EMBRYOLOGY 



resemblance, however, is one of general pattern only and does not carry 

 through into the finer details of structure. Furthermore, the embryonic 

 circulatory system changes from day to day as new structures develop and old 

 structures break up. During the development of the chick embryo, for exam- 

 ple, three types of excretory structures develop. First there is a pronephros 

 similar to that outlined for the frog embryo but nonfunctional. Next a 

 mesonephros develops, and this is the type of kidney found in the adult frog. 

 Finally the permanent kidney develops, and this is called a metanephros. 

 With the formation of the mesonephros the cardinal circulation is altered 

 to a renal portal circulation which resembles the renal portal circulation in 

 the adult frog. Finally, this renal portal circulation breaks down while the 

 mesonephros is degenerating and a new circulation is established with the 

 developing metanephros. We shall discuss the changes in the development 

 of the circulatory system in isolated regions and not attempt to follow all 

 changes simultaneously. 



EXTERNAL CAROTID 

 * / AORTIC ARCHES 



INTERNAL CAROTID 



ANTERIOR 

 CARDINAL 

 VEIN 



POSTERIOR 

 CARDINAL- 

 VEIN 



ALLANTOIS 

 UMBILICAL VEINS 



TAIL REGION 



FlG. 106. Circulation in the 

 early chick embryo. A ventral view 

 with the left side of the cardinal 

 system omitted. Blood flows from 

 a two-chambered heart through a 

 short ventral aorta into four pairs 

 of aortic arches, then up into col- 

 lecting arteries which join to form 

 a dorsal aorta. The forward exten- 

 sions of the arteries in the head re- 

 gion represent the carotid arteries, 

 which develop from the ventral and 

 dorsal remnants of the aortic arches. 



