DEVELOPMENT OF THE HEART IN MARSUPIALS. 473 



Stage] III. and at the anterior margin thereof in the present 

 stage. 



The first and second visceral pouches are now well marked 

 (fig. 5, V.P. 1 & 2), while the relations of the maxillo-mandibular 

 process and hyoid arch show little advance on the preceding 

 stage. 



The antero-median portion of the pericardium has increased 

 very considerably in the antero-posterior direction. Furthermore, 

 it may be noted that as the portion of the gut lying anterior to 

 the first visceral pouch has remained the same length (compare 

 figs. 4 & 5) and the anterior margin of the pericardium is now 

 situated in the same plane with the first visceral jDOuch, the 

 pericardium must have moved backwards as a whole. 



M M P 



J 



Text-figure 12. 



M^"^'" "'^iJr-- — -^ """ yy' 



Ferameles nasuta (IS, B). Longitudinal section, median through 

 the anterior end. 



A.I.P. Anterior intestinal portal. C.F, Cardiac fold. M.3I.P. Margin 

 of medullar}' plate. P. Pericardium. F.P. Protochordal plate. 



The form of the endothelial primordia of the heart is shown 

 in fig. 5. They have fused at their cephalic extremity, the fused 

 portion extending through some eighteen sections and represent- 

 ing the most closely approximating portions of the endothelial 

 tubes in Stage III. (PI. I, fig. 4). From it is derived the bulbus 

 (conus) arteriosus of the next stage (PI. II. fig. 8, B.A.). Posterior 

 to this fused portion, the endothelial tubes lie close together but 

 unfused for a considerable portion of their length (PI. I. fig. 5 

 and text-fig. 14), and then diverge widely and pass into vitelline 

 veins. The endothelial tubes throughout their length are 

 enclosed by the myocardial wall, which shows characteristic 

 thickening and prolongations of its cells throughout the greater 

 part of the length of the heart (text-fig. 14, My.). 



The myocardium of the right and left sides is united from the 

 cephalic extremity of the heart primordium to the point of 

 divergence of the right and left endothelial tubes, but the line 

 of fusion is not marked by any groove. In the posterior region 

 where the endothelial tubes separate from each other, each is 

 surrounded by its own myocardial layer, so that for a short 

 distance in front of the anterior intestinal portal, the two heart- 

 tubes lie below the closed gut, each surrounded by an indejDendent 

 fold of splanchnic mesoderm. The primordia of the heart 



