DEVELOPMENT OF THE HEART IN MARSUPIALS. 463 



progressive differentiation of angioblast cells in the cephalic 

 portions of the pleuro-pericardial canals. 



The evidence of this stage does not justify any definite state- 

 ment with regard to the origin of the endothelium of the heart. 

 From text-fig. 3 B it will be seen that the splanchnic mesoderm 

 [S.JI.) shows distinct traces of proliferative activity ; its ventral, 

 indented margin has an irregular outline and there are indications 

 of loosening of the cells. On the other hand, there is no definite 

 evidence of entodermal proliferation, though in an earlier stage, 

 which will form the subject of a separate paper, the appearances 

 by no means exclude the possibility of the entodermal origin of 

 the endothelium, whilst there is clear evidence of proliferative 

 activity on the part of the entoderm of the area vasculosa. 



(b) Dasyurios viverrinus (7*5 mm. vesicle). 



The material on which the following description is based 

 consists of two embryos, one cut transversely, the other longi- 

 tudinally. 



Greatest length of each embryo = 7 mm. 



In this stage there is a flat brain-plate, the anterior margin of 

 which is marked in the middle line by a thickened terminal ridge 

 (text-fig. 4, T.R.). There is a well developed neural crest proli- 

 feration, the anterior portion representing the primordium of 

 the trigeminal ganglion, and the posterior that of the facial, 

 glosso-pharyngeal and vagus ganglia. No somites are yet 

 difterentiated. 



The outline of the anterior end of the bi-ain-plate and the 

 pleuro-pericardial canals and endothelial heart-tubes are shown 

 in PI. I. fig. 2. From this it will be seen that the pleviro- 

 pericardial canals {P.p.G.) extend continuously round the head- 

 end of the embryo and lie anterior to the anterior margin of the 

 brain-plate (M.M.P.). 



The pleuro-pericardial canals attain their greatest size in the 

 hind-brain region [II. B.) at this stage, and here also the primordia 

 of the heart are well established in the form of endothelial tubes 

 lying between the entoderm and the thickened splanchnic 

 mesoderm. The endothelial tubes terminate anteriorly at the 

 level of the posterior limit of the trigeminal primordium. In 

 front of this, however, there are scattered angioblast cells and 

 strands of cells extending foi-wards as indicated in PI. I. fig. 2. 

 In the condition of the heart primordia the Dasyurus embryos 

 are in advance of those of Peravieles described above, for the 

 endothelium is definitely tubular throughout a great portion of 

 its extent and the myocardial fold is conseqviently well developed. 



From the longitudinal section (text-fig. 4) it will| be seen that 

 the antero-median portion of the pleuro-pericardial canal lies 

 some distance in front of the anterior margin of the brain - 

 plate {T.B.). 



In Stage I., then, we have a horseshoe-shaped pleuro-pericardial 



