DEVELOPMENT OF THE HEART IN MARSUPIALS. 



465 



Total length of embryo A, 8'5 mm. 



This embryo possesses a flat brain-plate with well marked 

 optic grooves ; somites are not as yet distinctly diflferentiated. 



The position of the anterior margin of the brain-plate is indi- 

 cated in fig. 3 {M.M.P.) as well as the outline of the gut {F.G.). 

 On comparison with fig. 2, it is clear that the progress of the 

 head-fold has brought about considerable alteration in the 



Text-figure 5. 



MM.e-, 



Dasyurv-s viverrinus (8"5 mm.). Longitudiual section of embiyo Aa, 

 median through anterior end. 



C.F. Cardiac fold. F.G. Fore-gut. M.M.P. Margin of medullary plate. 

 P.p.C. Pleuro-pericardial canal. 



relations of the gut, brain, and pericardium (see PI. I. fig. 3 and 

 text-fig. 5). In embryo A (fig. 3), which is slightly in advance 

 of embryo Aa (text-fig. 5), the anterior margin of the brain-plate 

 now marks the actual anterior limit of the embryo, while at the 

 same time the crescent -shaped fore-gut {F.G.) has developed. Its 

 anterior limit lies immediately behind the anterior margin of the 

 brain- plate, while closely applied to its posterior and latei'al 



