202 GENERAL BIOLOGY 



development. The passage leading forward from the ven- 

 tricle, destined to become the arterial trunk, becomes con- 

 joined with other paired passages in the mesoderm of .the 

 throat leading to the gills, which be- 

 come the branchial arches. Corre- 

 sponding vessels develop upon the 

 dorsal side and become conjoined 

 with the great dorsal aorta, leading 

 rearward (see figures 130^, and 131). 

 About the time these vessels are 

 first marked out, gills develop upon 

 the gill arches externally (fig. 126) 

 and become traversed by a system of 

 capillary vessels, which are at this 

 ^ffe"n e t s a d tio r n : of y c 'ham! stage the connecting link between the 

 co" s pie n t d on a o v f es 4e' s dorsal and ventral vessels just men- 

 s Tn^; fl aTu r &et,: e ven S - tioned. The circulation of the blood 

 tricle - through these transparent external gills 



is easily observed with the microscope, and it is a beautiful 

 sight. 



This simple type of circulation is essentially fish-like (fig. 

 130^). There are no lungs as yet, and hence there is no 

 pulmonary circulation. The heart is but two chambered. 

 All the blood passes forward from the ventricle through the 

 gills, to be returned rearward through the dorsal aorta. 

 The aortic arches are four, and at first essentially alike. 



Development of pulmonary circulation. Lungs develop 

 as already noted by outgrowth from the ventral pharyngeal 

 wall, and blood vessels to supply them arise from the fourth 

 branchial arch and extend rearward to penetrate their walls. 

 Return channels are developed, joining the lungs directly 

 with the heart. When these vessels become functional, a 

 considerable part of the blood is diverted from the gills to 

 the lungs. This, however, is of late occurrence, being an 



