Ch. XIV] ORGANS FROM THE MESODERM 151 



been described as originating from the ventral wall of the arch- 

 enteron, and if so, have had a different origin from the other 

 cells of the heart. 1 



At a somewhat later stage of development the walls of the 

 coelomic cavities of the right and left sides separate further 

 (Fig. 45, B). The splanchnic layer thickens, and begins to sur- 

 round the proliferation of scattered "endodermal cells." These 

 endodermal cells arrange themselves into a thin-walled tube 

 stretching throughout the heart-region (Fig. 45, B). Subse- 

 quent development shows that this tube becomes the endothe- 

 lial lining of the heart. Around this endothelial tube the 

 thickened splanchnic layers now begin to push in from the sides 

 between the tube and the lower wall of the pharynx. The tube 

 becomes finally entirely surrounded by mesoderm (Fig. 45, C). 

 The mesoderm from the sides that has met beneath the pharynx 

 forms the dorsal mesentery of the heart. The mesoderm around 

 the tube continues to thicken, and forms later the musculature 

 of the heart. 



At first the heart has also a ventral mesentery formed by the 

 union of the walls of the coelomic cavities below it (Fig. 45, B), 

 but later the mesentery is in part absorbed and the coelomic 

 cavities become continuous below from side to side, forming the 

 pericardial chamber. The outer layer of somatic mesoderm 

 gives rise to the pericardium itself. 



The tubular heart is attached at its posterior end to the 

 liver and anteriorly to the wall of the pharynx. It becomes 

 free ventrally and later also dorsally along the middle of its 

 course, and owing to an increase in length is bent on itself 

 into an c/>-shaped tube (Fig. 39). 



When the tadpole is 4-| mm. in length, we find a vessel open- 

 ing into the posterior end of the heart, the sinus venosus, 

 formed by the union of two large vitelline veins. These veins 

 have appeared on each side of the liver-diverticulum and con- 

 tinue along the yolk-mass in a fold of the splanchnopleure. 

 They are supposed to carry to the heart the food-material ab- 

 sorbed from the yolk. Into the sinus venosus empty also two "" 



1 At least these cells have arisen from the yolk-cells after the ventral meso- 

 derm has been split off. 



