392 EMBRYOLOGY OF THE LOWEE VERTEBRATES oh. 



series of chambers — sinus venosus, atrium, ventricle and conus 

 arteriosus — the originally peristaltic waves of contraction tending to 

 become reduced at each constriction so that the chambers come to 

 contract in series. 



(3) The primitive valvular apparatus consists of a series of 

 longitudinal ridges. These are best marked in the conus where 

 they were originally at least four in number. The presence of six 

 in Polypterus and the occurrence of more than four rows of valves 

 in various other relatively primitive Ganoids and Elasmol iranchs 

 suggests that the number may have been greater. Individual ridges 

 may in various forms become reduced or disappear, while in otber 

 cases apparently an increase in number may take place. The ridges 

 show very generally a tendency to develop from two distinct rudi- 

 ments, an anterior one and a posterior one, but the continuity of the 

 rows of valves in the lower fishes indicates the probability that this 

 discontinuity is secondary. In the portions of the heart behind the 

 conus there is no complete series of ridges like those in the conus 

 though it is possible that vestiges of such ridges are represented by 

 the endocardiac cushions and septal rudiments. 



(4) Using the ridges of the conus as marking morphologically 

 longitudinal lines it is seen that the conus has in the Lung-fishes 

 undergone a double flexure of such a kind as to produce when 

 straightened out a right-handed twist of the conus through approxi- 

 mately three right angles. 



(5) In tetrapodoua Vertebrates the conus shows a similar right- 

 handed twist and this is adequately explained by the relative reduc- 

 tion in length which the conus has undergone if it be assumed that 

 there was an ancestral condition resembling that of the existing 

 Dipnoan. 



(6) From the Lung-fishes upwards the originally right-hand ridge 

 of the conus becomes hypertrophied and, either alone or by fusion with 

 its ms-h-vis, forms a longitudinal septum dividing the cavity of the 

 conus into two parts — pulmonary and aortic — twisted spirally round 

 one another. 



(V) Correlated with this process is a tendency for the wall of the 

 conus to lose its striped muscle and its rhythmic contractility. 

 This process takes place from before backwards and the portion 

 which has suffered this change, and the wall of which has assumed 

 the ordinary arterial character, is in common usage included under 

 the name truncus arteriosus. 



(8) The atrioventricular . part of the cardiac tube undergoes a 

 double flexure similar in nature to that seen in the conus of the 

 Lung-fish but of such a kind as would if straightened out give a 

 left-handed twist. 



(9) The valves and septa of this part of the heart originate in 

 the endocardiac cushions and in Zepidosiren atrial septum and ventri- 

 cular septum are at first continuous with one another. 



(10) In the evolution of the valves of the heart there has come 



