DEVELOPMENT OF THE HUMAN HEART 261 



fibrils are heaped up into two mounds to form the well known 

 endocardial cushions, which have between them a transverse 

 slit. The posterior cushion extends upward into the left atrium 

 and then along its posterior surface into the right atrium and ends 

 at the opening of the sinus venosus. The anterior cushion also 

 extends into the left atrium along its anterior border and reaches 

 to the septum primum which is just beginning to form. Below, 

 in the left ventricle, the endocardial cushions blend with the endo- 

 thelial reticulum covering the trabeculae. The interlacement of 

 the endothelial and muscular layers to form the trabeculae extends 

 into the right ventricle, but in the bulb the two layers are quite 

 sharply defined and separated. 



The nuclei of the endothelial syncytium form first of all the 

 inner layer of the heart, but in the endocardial cushions of the 

 atrial canal as well as in the bulb the nuclei gradually extend 

 towards the muscular coat. In other words the nuclei of the inner 

 coat are gradually invading their reticular layer. 



In the heart of embryo 463 the differentiation of the endothe- 

 lial syncytium is more pronounced than in the specimen just 

 described. The heart is now well formed with two pronounced 

 atria, a much constricted atrial canal and a marked constriction 

 of the interventricular canal (fig. 4). The bulbo-ventricular and 

 the interventricular grooves are well formed. The septum of 

 the atria and that of the ventricles are well marked. The endo- 

 thelial syncytium is most pronounced in the endocardial cushions 

 and in the bulbus. The cells are quite equally distributed through- 

 out the syncytium but they are somewhat more numerous imme- 

 diately under the endothelial covering than near the muscle layer 

 of the heart (fig. 6). The posterior cushion does not reach as 

 far into the left ventricle- as the anterior and is also less extensive 

 in the atria; it reaches nearly to the sinus venosus. The anterior 

 endocardial cushion is a large sickle-shaped affair, encircles the 

 heart in front as the border of the atrial septum (septum primum ) 

 which is now forming. The anterior cushion ends on the medial 

 side of the opening from the sinus venosus. The space in the 

 atria between the cushions marks the primary foramen ovale (fora- 

 men ovale I). 



