DEVELOPMENT OF THE HEART IN MARSUPIALS. 



469 



towards the lateral margin of the gut and then parallel with!this 

 margin (PI. I. fig. 4, A I). It loops round the anterior limit of 

 the gut, joins the dorsal aorta, and thus constitutes the first or 

 mandibular aortic arch. It is impossible here to fix any exact 

 limit between the endothelial heart-tube and the aortic arch.^ 

 Text-figs. 7 and 8 represent typical sections through each. The 

 transition from the heart primordium to the aortic arch is 

 indicated by the gradual reduction in the dorsi-ventral extent of 

 the space surrounding the endothelial tube and the rotation 

 of the vessel, so that its greatest diameter comes to lie parallel 



Text-figure 8. 



Perameles nastita (1 S, A.). 



A. Transverse section through heart primordia and second aortic arch. 



B. Complete outline of same section. 



A-2. Second aortic arch. I). A. Dorsal aorta. ISiid. Endothelium of heart.- 

 M.m.A. Maxillo-mandibular arch. M.P. Medullary plate. My, Myocardium. 

 P. Pericardium. V.C.M. Vena capitis medialis. 



with the floor of the gut. (Compare text-figs. 8 and 7.) Outside 

 the limits of the pericardium the aortic arch runs in the mesoderm 

 of the maxillo-mandibular process (text-fig. 7). It is connected 

 with the dorsal aorta by one main loop and by several smaller 

 vessels which were omitted from fig. 4 for the sake of clearness. 



The second vessel, which arises from the anterior end of the 

 heart, is small and runs backwards and outwards, lateral to and 

 almost parallel with the heart-tube, and has precisely the same 

 relations to the gut and mesoderm. 



