Mammalia: Visceral Systems 629 



orders which still survive. It is highly probable that this divergence 

 loward the mammalian type involved not only features of skeleton 

 and teeth but also the organs of respiration and circulation. Along the 

 line initiated by the theromorph reptiles must have appeared mi- 

 nutely alveolar lungs and a vocal laryngeal organ of the mam- 

 malian type, and an arterial pattern whose key feature is persistence 

 of the left aortic trunk as sole outlet from the left ventricle and the 

 only connection between heart and dorsal aorta. Along other lines, 

 the lungs acquired the coarsely saccular structure seen in modern 

 reptiles. One of these lines, its early history quite unknown to us, must 

 have witnessed the transformation of saccular into tubular lungs, the 

 development of the ultrapulmonary avian air-sacs, and the shifting 

 of the function of vocalization to a syrinx at the posterior end of the 

 trachea. Meanwhile, the arterial pattern became that which is charac- 

 terized by the crossing of the right and left trunks of the fourth aortic 

 arch and the concentration of functional importance on the right 

 trunk, whose connection is with the left ventricle. The term "Saurop- 

 sida" was proposed by T. H. Huxley to include all reptiles and birds. 

 It has more recently been suggested that it would be better to restrict 

 the term to include birds, all modern reptiles, and extinct reptiles 

 exclusive of those which were in the main line of mammalian descent. 

 Then the mammals, together with their reptilian progenitors beginning 

 with the theromorphs, could be designated as the "Theropsida" 

 (meaning "mammal-like"; not to be confused with "Therapsida," 

 a group of theromorph reptiles) . 



The persistence of the right instead of the left trunk of the em- 

 bryonic fourth arch in a mammal results in a condition which looks 

 quite avian. But it cannot be imagined that there is any genetic or 

 causal relation of any sort between the abnormality and the condition 

 in birds. Certainly there have been no birds in the mammalian "family 

 tree." The presence of both right and left vessels of the fourth arch in 

 an adult mammal is deceptively reptilian, but the fact that both the 

 right and the left vessel come off by a common trunk from the left 

 ventricle absolves the animal of any suspicion of being a reptilian 

 "throwback." The fact that in all Classes of vertebrates the early 

 embryo possesses a simple and longitudinally undivided heart and 

 ventral aorta and a set of aortic arches makes possible such abnormali- 

 ties as these. Their occurrence must be due to some derangement of 

 the developmental mechanism, the nature and cause of it unknown. 

 Any resulting resemblance to the vascular pattern of some other Class 

 is probably merely incidental and may not safely be interpreted as a 

 case of reversion so long as the abnormality involves only a single 

 element of the complex pattern. 



