DEVELOPMENT OF THE HEART IN MARSUPIALS, 



477 



The first and second visceral pouches are seen in the model as 

 lateral projections from the gut (figs. 6-8, V.P. 1 & 2). The 

 heart, which is median and ventral anteriorly, still consists of 

 separate lateral primordia posteriorly, the two halves diverging 

 in the lateral lips of the anterior intestinal portal (fig. Q,A.I.F.). 

 By the great enlargement of the heart itself, the pericardial coelom 

 has become relatively considerably reduced, and now simply forms 

 a space surrounding the heart ventrally and ventro-laterally 

 (text-fig. 18). The median pericardium extends from the 

 cephalic extremity of the heart to the anterior intestinal portal. 



At their cranial ends the endothelial tubes (exposed in the 

 model by the omission of the myocardium) unite to form a broad 

 conical portion, the bulbus (conus) arteriosus. The first and 

 second aortic arches (A. 1 & 2) are given ofl:' from the dorsal side 

 of the bulbus (fig. 8, £.A., A. 1 &c.). In this region, the myocar- 

 dium simply forms a continuous layer covering the endothelium, 



Text-figure 16. 



Ferameles ohesula (10.viii.03). 



A. Transverse section through bulbus arteriosus. 



B. Complete outline of same section. 



End. Endothelium of bulbus arteriosus. G. Gut. My. Myocardium. 



but separated from it by a space (text-fig. 16). The bulbus is the 

 only portion of the heart in which the endothelial tubes have 

 actually fused. Behind it, the tubes are in contact as far back as 

 the anterior iiitestinal portal, but the wall between them is every- 

 where complete. The myocardium of the two sides has fused 

 throughout the region of the closed gut and there is no ventral 

 mesentery (text-figs. 16-18). There is, however, a very well 

 marked groove on the ventral aspect of the myocardium, which 

 marks the line of junction of the right and left halves. At the 

 opening of the gut the right and left heart primordia separate 

 completely. 



From the ventral view of the model (PI. II. fig. 6) it is obvious 

 that already the heart has begun to bend between the two points 

 la) the roots of the aortic arches and (&) the opening of the fore-gut 



33* 



