486 miss k. m. paeker on the 



Summary and Discussion, 

 A. Development of the Heart. 



From the foregoing description it is evident that the early- 

 development of the heart in such Marsupials as Perameles and 

 Dasyurus proceeds along essentially the same lines as in Eutheria. 

 The early stages of the heart development in the latter have been 

 described by a number of investigators {e.g. Mollier (15) ) ; but 

 although the broad outline of the process may be said to be well 

 known, there is still considerable difference of opinion with regard 

 to certain points. It will be useful, therefore, before summarising 

 the preceding observations, to briefly review the literature on the 

 subject. 



With regard to the lateral paired primordia of the heart little 

 need be said at this point. The heart endothelium arises between 

 the entoderm and the splanchnic mesoderm, from which latter 

 it is either partially or wholly derived. The primordia of the 

 heart-tubes are first recognisable in the hind-brain region and 

 grow forwards at the expense of angioblastic cells proliferated off 

 from the splanchnic mesoderm, which is itself thickened and 

 indented to form the primordium of the myocardium. It should 

 be noted that in the earliest stages examined (PI. I. figs. 1 & 2) 

 the endothelium lies to the medial side of the pleuro-pericardial 

 canals in the anterior region and to the lateral side in the pos- 

 terior region. To this point reference will be made subsequently 

 in connection with the discussion of the reversal of the peri- 

 cardium which, according to some authors, takes place at the 

 time of formation of the head-fold. 



The processes which bring the lateral heart primordia into 

 position below the fore-gut relate primarily to the formation of 

 the head-fold, and it is therefore necessary to get a clear idea of 

 the mode of closure of the gut before considering the problems 

 relating to the fusion of the lateral primordia of the heart. 



Some authors (e. g. Robinson (13) ) hold that the formation of 

 the fore-gut is due mainly, if not entirely, to the rapid growth 

 of the embryo over the relatively stationary line between the 

 embryonal and extra-embryonal areas. Thus Robinson (13) says : 

 " The orifice (of the umbilicus) is not reduced in size during the 

 early stages of development by the convergence of its margins 

 towards a central point. This being the case, no tucking off of 

 the embryo from the surface of the ovum can occur ; on the con- 

 trary, what does occur is almost the exact opposite of such a 

 process, for the margin of the area remains as a relatively 

 slow-growing region, whilst the embryonic and extra-embr^^onic 

 portions of the wall of the ovum rapidly increase in extent. 

 Under these circumstances, it follows that the margin of the 

 embi-yonic area will soon appear as a ring between the upper or 

 embryonic and the lower or extra-embryonic parts of the ovum, 

 both of which have expanded beyond it in all directions," 



