DEVELOPMENT OF THE HEART IN MARSUPIALS. 



479 



curvature. (See PI. II. fig. 6.) Correlated with this difference in 

 the size and curvature of the endothelial tubes, the opening 

 of the fore-gut is also asymmetrical (fig, 6, A.I.P.). At the 

 anterior intestinal portal, the right and left primordia of the 

 heart separate, and both tubes become reduced in size, the right, 

 however, more markedly than the left. The endothelial heart- 

 tubes pass imperceptibly into vitelline veins. 



A further distinction between the ventricular and auricular 

 portions of the heart lies in the fact that in the anterior region 

 the myocardium is separated from the endothelium by a con- 

 siderable space crossed by fine strands of protoplasm (text-fig. 17), 

 while in the posterior portion the myocardium is closely applied 

 to the endothelium (text-fig. 18). The transition between these 

 two conditions takes place gradually in the region of the atrio- 

 ventricular constriction of the right side. 



Text-figure 18. 



Perameles obesula (10.viii.03). 



A. Transverse section through auricular portion of the heart. 



B. Complete outline of same section. 



End. Endothelium. G. Gut. Mi/. Myocardium. 



Turning now to the blood-vessels, two aortic arches are complete. 

 Their relations are seen most clearly in the side view of the model 

 (PI. II. fig. 8, ^. 1 & 2). From the ventral view (fig. 6) it will be 

 seen that there are a number of capillaries lying against the gut- 

 wall, between the roots of the right and left mandibular arches. 

 These are probably derived from the scattered angioblast cells in 

 the corresponding position in Stage III., and are doubtless destined 

 to contribute to the formation of the median ventral aorta which 

 is established in the next stage (Stage VI.). The first aortic arch 

 runs forwards to the anterior end of the gut and is there connected 

 by a well developed loop, situated laterally to the apex of the 

 fore-gut, with the corresponding dorsal aorta. The aorta is paired 



