VI 



HEART OF ELASMOBRANCHS 



375 



such simple mechanical explanations of developmental phenomena 

 with suspicion. Similarly the atrioventricular cushions become 

 excavated on their ventricular side and form the two atrioventricular 

 valves of the adult. A pair of laterally placed valves also develop 



Fig. 179.— Views of the heart of Lepidosiren as seen from the morphologically ventral side. 

 (B and C after J. Robertson, 1913.) 



A stage 32 ; B, stage 31 ; C, stage 35. at, atrium ; C, conus arteriosus ; La, left auricle ; r.a, right 



auricle ; V, ventricle. 



at the opening of sinus into atrium but according to extant descrip- 

 tions (Rose, 1890) arise merely as infoldings of the heart wall. 



D IPNO i. The development of the dipnoan heart has formed the 



subject of a careful and exhaustive study by Robertson (1913) and 

 her account of the development of the heart in Lepidosiren forms 

 the basis of the following description. 



