On the Classification of the Reptilia. 



271 



Crocodile (D), the hearts are represented as untwisted, so as to bring the 

 chambers back into a single plane and facilitate comparison. 



Now in the Amphibian a horizontal septum grows back, subdividing the 

 lumen of the truncus into dorsal and ventral channels, and combines with 

 one of the distal valves to form an oblique septiim in the bulbus cordis, 

 which septum directs most of the arterial blood into the ventral channel 

 leading to the systemic and carotid arches, and most of the venous blood 

 into a dorsal channel to the pulmonary arches. In the Amniota the valves 

 are fully developed only in that narrow posterior region at the base of the 

 bulbus, which becomes incorporated into the wall of the ventricle. The 

 lumen of the truncus and bulbus becomes completely and spirally subdivided 

 into two tubes, the pulmonary and the systemic or aortic. As shown by the 

 work of Greil (15) and others, this is brought about by the completion of 

 the horizontal septum with the help of the anterior valves of the bulbus. 

 But whereas in the Mammalia the interventricular septum is so formed that 

 the right (venous) ventricle leads only into the pulmonary artery, and the 

 left (arterial) only into the aortic arch and carotids, in the Eeptilia the 

 interventricular septum tends to divide the chamber into a left cavity leading 

 to the base of the right systemic arch, and a right cavity leading to the base 

 not only of the pulmonary, but also of the left systemic arch. Thus, when 

 the septum is completed, as in Crocodiles and Birds, the right ventricle 

 opens into the pulmonary artery and left systemic arch, while the left 

 ventricle opens into the right systemic arch. The two systemic arches cross 

 over at their base, and the main arterial stream is always sent up the right 

 arch, from which spring the carotids. The fundamental difference lies in the 

 subdivision of the Sauropsidan bulbus down to its very root into two separate 

 spirally twisted tubes, one crossing to the left and the other to the right, in 

 such a way that the interventricular septum comes to pass between them. 

 This line of specialisation inevitably leads in the long run to the Avian 

 type, where the left systemic arch — already of little use in the Crocodile — 

 disappears early in development. In the Mammal, on the contrary, the 

 aortic trunk, separated from the pulmonary, never becomes subdivided at 

 all, and the differentiation of the arteries has followed an independent and 

 in many respects different course from the primitive bilaterally symmetrical 

 pattern. The Theropsidan and the Sauropsidan types must have evolved from 

 some more symmetrical primitive type, in which the ventricle and the aortic 

 trunk were both single ; and it does not seem possible for a heart which had 

 once started, so to speak, to evolve along the Sauropsidan line to change its 

 course and revert to the Theropsidan. The significance of this in determining 

 the phylogeny of the Chelonia and the Lacertilia may now be pointed out. 



