270 Vertebrate Embryology 



the two sets of semilunar valves have been 

 formed between the two divisions of the trun- 

 cus and the two ventricles into which they 

 open. 



During the next two or three days the heart 

 continues to approach more nearly the shape 

 of the adult, though there is no remarkable 

 change in structure. 



By the twelfth day, the interauricular sep- 

 tum has increased to such an extent that the 

 opening between the two auricles is reduced 

 to a small opening, the foramen ovale. 



Shortly before hatching, the foramen ovale 

 becomes nearly closed by a membranous fold ; 

 and shortly after hatching, the closure is com- 

 plete, and the heart has practically the adult 

 structure. 



During the later stages of incubation, the 

 walls of the ventricles and auricles, especially 

 the former, become much thickened by the in- 

 ward growth of ridges from the muscular walls, 

 forming the trabeculae of the adult heart. The 

 truncus becomes thickened simply by the in- 

 crease in the thickness of the component 

 layers of its walls. 



The pericardial and pleural cavities. It will 

 be seen, by the examination of figures of the 



