388 EMBEYOLOGY OF THE LOWER VERTEBRATES ch. 



The pocket -valves are stated not to develop at the extreme 

 hinder limit of the ridges, the septum stretching back beyond them 

 to become continuous with the interventricular septum. 



In the right auriculoventricular opening the inner or septal 

 valve is not developed, the ventricular septum fusing with what in 

 Lacerta becomes converted into the valve in question. 



The main features of heart development having been illustrated 

 from these three different groups, Elasmobranchii, Dipnoi and 

 Sauropsida, it will be convenient now to indicate the more important 

 peculiarities which have been detected in other groups of the lower 

 Vertebrates. It should be understood however that in the case of 

 several of these, such as Cyclostomes, Ganoids and even Amphibians, 

 apart from Urodeles, our knowledge is still fragmentary. 



In Polypterus (Graham. Kerr, 1907) the cardiac tube when in the 

 form of a loop shows a similar displacement to that which occurs 

 in Lepidosiren — the lower end of the loop being pushed forwards in 

 front of the yolk. In this case however the displacement has gone 

 farther than in the Lung-fish so that the cardiac loop is completely 

 inverted — its apex being directed forwards, while the ventral aorta 

 passes off in a tailward direction. A similar displacement occurs in 

 Teleosts. 



The conus of Ganoid fishes shows the usual endocardiac ridges 

 which become converted into longitudinal rows of pocket-valves as 

 in Elasmobranchs. In Polypterus these ridges are six in number, 

 alternate ones being much reduced in size, with the result that in 

 the adult three rows of large pocket-valves alternate with three 

 rows of small ones. In all these fishes the endocardiac ridges and 

 their resultant rows of pocket-valves run straight along the conus 

 and there is no reason to doubt that this is the primitive condition. 

 To determine the primitive number of the ridges in Fishes more 

 research is needed although there is little doubt that four was the 

 number present in the primitive Tetrapods. 



In the Teleostean fishes we find in place of the conus arteriosus 

 the structure known as the aortic bulb. As already indicated 

 (p. 373) this is distinguished from the typical eonus by well-marked 

 histological and physiological differences. And it is frequently 

 regarded as being morphologically a part not of the heart but of the 

 ventral aorta. 



If however we take, as we are probably justified in doing, the 

 point of exit from the pericardiac cavity as being relatively fixed 

 and as marking the headward limit of the cardiac tube or primitive 

 heart, then it becomes clear that the aortic bulb, lying as it does 

 within the pericardiac cavity, is really a portion of the primitive 

 cardiac tube and of that part of it which lay between the ventricle 

 and the ventral aorta — in other words the conus arteriosus. What 

 has happened in the evolution of the Teleostean heart is in all 

 probability entirely analogous with what has taken place in the 



