DEVELOPMENT OF THE HEART IN MARSUPIALS. 483 



two embryos. In the Maoropus embryo, as in Ferameles, the 

 myocardium is fused in the middle line throughout the length of 

 the closed gut. A slight groove mai-ks the line of fusion in the 

 posterior portion of the heart, but there is no indication of a 

 ventral mesentery at any point. The ventricular region of the 

 heart is distinguished from the auricular by the fact that in 

 "the ventricular portion a considerable space intervenes between 

 •endothelium and myocardium, whereas in the auricular portion 

 the two layers are close together. The limit between the two 

 divisions is further indicated by a constriction of each endo- 

 thelial tube, which then widens out abruptly to form the auricle. 

 Right and lefh endothelial tubes are united anteriorly in the 

 region of the bulbus arteriosus, just as in the Perameles embryo. 

 JBehind this again they diverge around the opening of the gut. 

 It may be noted that in the region of the widest divergence of 

 the endothelial tubes a fine bridge runs across and connects 

 the two. 



The curvature, so far as it can be made out without reconstruc- 

 tion, is similar to that of the Perameles embryo. The asymmetry 

 appears to be less marked than in the latter embrj^o, but on this 

 point it is impossible to make a positive statement without recon- 

 struction. Throughout a considerable portion of its length, 

 however, the right endothelial tube is lax^ger than the left, just 

 as in the Perameles embryo. 



The Macropus embryo then differs from the Perameles embryo 

 of a similar stage mainly in the configuration of the endothelial 

 tubes, which are joined at their cephalic extremity, then widely 

 separate for some distance (text-fig. 21), and then again approxi- 

 mated, though not joined. This difference, as well as the slight 

 differences in the myocardial wall, may very probably be due to 

 slight dissimilarity in the positions of the endothelial tubes and 

 the myocardium before union of the latter. 



Stage VI. Macropus ruficollis. 



The material for this stage consists of a single embryo of 

 Macropus ruficollis, cut transversely. 



Greatest length of embryo, 5'2 mm. 



Dorsal perimeter, about 13'5 mm. 



The embryo is sharply bent in front of the fore-limb buds, so 

 that the head, invested by proamnion, is sunk doAvn into the yolk- 

 sac and forms an acute angle with the trunk. No trunk amnion 

 is yet developed. The brain, though open in the fore- and mid- 

 brains, is closed in the region of the hind-brain. 



The gut is closed as far back as the third well-developed somite. 

 Three visceral pouches are present. 



The heart shows a considerable advance on the preceding stage. 

 The right and left heart-tubes are fused except in the region of 

 the sinus venosus, where they remain separate, while the Cuvierian 



