VI 



iii-Airr <>r ELASMOBRANCHS 



375 



such simple mechanical explanations of developmental phenomena 

 with suspicion. Similarly the atrioventricular cushions become 

 excavated <>n their ventricular side and form the two atrioventricular 

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



r.a. 



ra 



i 



la 



V 



FIG. 179. Views of the heart of />/</</".<//> a- >ci/n from the morphologically ventral side. 

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



A, sta^e :i-J ; H, st;ip- ::i ; ('. sta^r y:>. <it, atrium ; r, conns artcriosns ; /.a, left auricle ; r.a, rij,'lit 



auricle ; F, \cntricle. 



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

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



DIPNOI. 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. 



