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Mabel Bishop 



arches or parts of arches sacrificed. As text figure 1 represents the nncler- 

 Iving condition in normal mammals, so also text figures 2 and 3 represent 

 the underlying condition in Teras XII and XV respectively, for as soon 

 as the arches began to appear in the Terata they were in the form repre- 

 sented in the diagrams. In other words, they did not make their appear- 

 ance as in a normal mammal and later acquire duplication, but on the 

 contrary, the doubling of the arches was present from their beginning. 



raa , \ 



Text Fig. 1. Diagram showing tlie trausforiuations in the primitive 

 arterial arches in normal mammals. 



1, 2, .3, 4, 5, 6, The six primitive arterial arches ; ext. car., External 

 caroticls; int. ear.. Internal carotids; H., heart; p., Pulmonary artery; 

 b., Ductus Botalli; s. el., Subclavian; V., Vertebral artery; raa., Right 

 aortic arch; laa., Left aortic arch; da., Dorsal aorta; mcec, Median com- 

 pound external carotid (Teras XII) ; ab., Aortic bulb divided (Teras XV) ; 

 X., communicating branch between components A and B (Teras XX). 



At the stage which the diagrams represent the heart is still a median 

 straight tube, its most posterior portion being the sinus venosus, the next 

 anteriorly the common atrium which communicates freely with the 

 common ventricle, and most anteriorly the aortic bulb which is con- 

 tinued into the aortic trunk. The aortic trunk bifurcates, each moiety 

 arching backward to the median axis of the body posterior to the heart, 



