DEVELOPMENT OF THE HEART IN MARSUPIALS. 487 



While the forward growth of the brain-plate doubtless plays 

 an important part in the initiation of the formation of the fore- 

 gut, this explanation does not account for the conditions revealed 

 by reconstructions of the gut and pericardium in the early stages 

 of head-fold formation. 



On the other hand, various investigators (e. g. Rouviere (14), 

 Graper (4) ) contend that there occurs, in addition to the forward 

 growth of the brain-plate, a backward progression of the anterior 

 intestinal portal, whilst in older accounts an actual fusion of 

 lateral folds in the mid-ventral line was assumed. Both Robinson 

 (13) and Rouviere (14) give excellent reasons for regarding this 

 assumption as erroneous. They point out that if gut-closure 

 were effected by the fusion of lateral folds (such as are shown in 

 text-fig. 9), the heart would remain in connection with the gut by 

 a dorsal mesocardium and with the yolk-sac wall by a ventral 

 mesocardium. Robinson denies the existence of a ventral meso- 

 cardium in mammals, and quotes this fact in support of his theory 

 that the separation of the gut from the yolk-sac is due to growth 

 of the embryo rather than to fold-formation. Rouviere, on the 

 other hand, while he agrees with Robinson as to the absence of a 

 ventral mesocardium in mammals, gives a different account of 

 the process of gut-closure. He describes the formation of lateral 

 pleuro-pericai'dial canals which grow forwards round the anterior 

 end of the brain-plate and fuse to form a continuous cavity. 

 The splanchnopleure forming the posterior wall of the pleuro- 

 pericardial cavity now forms a contihvious fold which Roixviere, 

 following Tourneux, calls the "cardiac fold" (compare text-fig. 12, 

 C.F.) and which he describes as growing actively backwards as 

 a whole. 



In the chick, on the other hand, a ventral mesocardium is 

 present, but this is due, as Robinson points out, to the relatively 

 late penetration of the mesoderm in the head region. The pleuro- 

 pericardial canals do not extend round and unite in front of the 

 medullary plate in early stages, but only at a later stage do they 

 penetrate into the floor of the fore-gut after that has been formed. 

 The lateral cavities therefore do not at once become continuous, 

 but remain separated from each other by a double layer of 

 mesoderm constituting the ventral mesocardium. 



With regard to mammals, Rouviere, while he does not discuss 

 the influence of the forward growth of the brain-plate, concludes 

 that the crescent-shaped cardiac fold grows backwards as a whole, 

 and that the free edge of the splanchnopleural fold progresses 

 always in advance of the primordia of the heart, so that no fusion 

 of the splanchnopleure is involved and no ventral mesocardium 

 is formed. 



Graper, in a description of the growth processes in the 

 developing chick, which he worked out by staining the living 

 embryos and keeping them under observation while still alive, 

 shoAvs that there is considerable evidence in support of the view 

 that the margin of the fore-gut (anterior intestinal portal) moves 



