MORPHOGENESIS OF CIRCULATORY SYSTEM 753 



valve, while the oxygenated blood passes ventrally to the spiral valve and 

 into the arteries coursing toward the head and into the systems (fig. 3351). 

 This condition of the conus is present also in urodeles with well-developed 

 lungs, but, in urodeles without well-developed lungs, the spiral valve is absent 

 and the interatrial septum may regress (Noble, '31, pp. 187-194). 



3) Amniota. The heart of reptiles, birds, and mammals differs from the 

 heart of the Amphibia in that a mechanism is present which separates, more 

 or less completely, the oxygenated blood from the non-oxygenated blood. 

 For example, the heart of birds and mammals is a four-chambered affair as 

 an interventricular septum divides the primitive ventricle into two separate 

 compartments while an interatrial septum separates the primitive atrium into 

 two atria. A double heart is produced in this manner wherein the non- 

 oxygenated blood returning from the organ systems passes through the right 

 atrium and ventricle en route to the lungs while the oxygenated blood from 

 the lungs journeys through the left atrium and ventricle on its way back to 

 the organ systems. In the heart of birds and mammals, it is to be observed 

 also, that only two arterial channels convey blood from the heart; namely, 

 a pulmonary arterial trunk and a systemic arterial trunk. Another feature is 

 present in the heart of the birds and mammals which serves to distinguish it 

 from the hearts found in all lower vertebrates, in that the sinus venosus is 

 absorbed almost entirely during embryonic development into the wall and 

 structure of the right atrium. 



Turning now to a consideration of the hearts of reptiles we find that the 

 turtles and snakes possess a heart with two atria and a ventricular region 

 divided rather completely into two ventricles. However, the interventricular 

 septum is slightly incomplete in the region near the atria, and some leakage 

 of blood between the two ventricles is possible. In the crocodilians the inter- 

 ventricular septum is completely developed, but a small opening, the foramen 

 of Panizza, is present at the bases of the two systemic arterial trunks. This 

 foramen arises as a secondary perforation later in development and does not 

 represent an incompleteness of the interventricular septum. In the reptilian 

 heart the sinus venosus retains its identity as a separate chamber of the heart. 

 Furthermore, contrary to the conditions found in the avian and mammalian 

 heart, three arterial trunks convey blood away from the ventricles. Two of 

 these vascular trunks come from the right ventricle, and one from the left 

 ventricle, for a pulmonary trunk conveys blood from the right ventricle to 

 the lungs, while a systemic aortic root also carries blood from the right ventricle 

 to the abdominal aorta. From the left ventricle, on the other hand, blood is 

 propelled through a single aortic root to the head, forelimbs, and abdominal 

 aorta (fig. 341H). 



a) Heart of the Chick. The heart arises as a simple tube during the 

 second day of incubation (fig. 339G). At the end of the second day and 

 during the third day, the primitive ventricle bends to the right, and the atrium 



