DEVELOPMENT OF THE HEART IN MARSUPIALS. 



489 



brain-plate has occurred, the pericardium has actually closed 

 in towards the axial line. Precisely the same conclusion may be 

 reached from a comparison of figs. 1 and 4, but as the interval 

 between the stages is greater and the embryos ai-e not of the 

 same species, less importance attaches to them in this con- 

 nection. 



It has alread}"- been remarked that, in the chick, the develop- 

 ment of the pleuro-pericardial canals occurs at a later period 

 than in the mammal, so that in this type the form of the head- 

 fold in early stages cannot be affected by growth of the 

 pericardium. If we compare the shape of the fore-gut in a chick 

 of two somites with that of Dasyurus Stage II. (text-fig. 23 A & 

 PI. I. fig. 3), we see that in the first stage of head-fold formation 

 in the bird, the outline of the anterior intestinal portal is broadly 



Text-figure 23. 



M.M.R 



A.LR. 



A.I.P 



Anterior end of chick of (A) 2 somites, (B) 4 somites, to show the relations of the 

 head-fold, brain-plate, anterior intestinal portal and pleuro-pericardial 

 cavities. 



A.I.P. Anterior intestinal portal. J?'. G". Fore-gut (outline). Jf.F. Medullarj-fold. 

 M.M.P. Margin of the medullary plate. F.p.C. Pleuro-pericardial cavity. 



crescentic, while in Dasyurus it is U-shaped. This difference 

 I conceive to be due to the fact that in the chick, no factor but 

 tlie forward growth of the brain-plate is operating at this stage, 

 while in the mammal, in addition to this process, the expansion 

 of the pericardium is already bringing about the formation of 

 lateral folds and the consequent narrowing of the anterior 

 intestinal portal. A slightly later stage of the chick (text-fig. 

 23 B) shows an approximation to the mammalian condition, for 

 the pleuro-pericardial canals have appeared and are progressing 

 towards the middle line ; lateral folds have therefore arisen and 

 the outline of the anterior intestinal portal is U-shaped. 



We may therefore conclude, that while the forward growth of 



