DEVELOPMENT OF THE HUMAN HEART 257 



Iii the youngest embryos studied the reticular mass between 

 the endothelial cells and the muscle wall appears either homogene- 

 ous, or as composed of most delicate fibrils, or of coarser fibers, 

 according to the method of preservation. In general it appears 

 like the most delicate reticulum of the mesenchyme and under all 

 circumstances any stain which tinges the fibrils tinges also the 

 endothelial cells. So intimate is this connection that it forces 

 the conclusion that the fibrils together with the endothelial cells 

 form a syncytium. In the younger hearts the endothelial nuclei 

 lie altogether on the inner side of the fibrils as has been repeatedly 

 observed, but as soon as the trabecular system begins to form in 

 the left ventricle some of the endothelial nuclei invade the fibrillar 

 layer. This is first seen in embryo No. 239 (4f mm. long). Here 

 the trabecular system is quite completely formed in the left 

 ventricle by an interlocking of processes from the muscular and 

 endothelial layers. In the right ventricle the process is not so far 

 advanced, while in the atrial canal and the bulb the reticular 

 layer is invaded by endothelial nuclei but not by muscle cells. 

 A similar arrangement is found in embryo No. 3, which as No. 

 239, is intensely stained. Fig. 5 is from the posterior endocardial 

 cushion of an embryo 4.3 mm. long, (No. 148), showing that the 

 nuclei of the cushion are invading it from its endothelial side. All 

 this is more pronounced in No. 463 (3.9 mm.) which is more ad- 

 vanced in development and is perfectly preserved. In this speci- 

 men it is quite easy to demonstrate that the nuclei of the endothe- 

 lium and the reticular mass belong together, for they are distinctly 

 intermingled and yet are separated from the muscular layer (fig. 

 6). Since the nuclei and fibrils belong together and since it has 

 been demonstrated that the reticulum of the liver is developed 

 from the endothelial cells, I shall speak of the reticulum between 

 the endothelium and muscle layer of the heart as endothelial 

 fibrils. The great importance of this distinction is at once appar- 

 ent for it shows that connective tissue arises also from endothelial 

 cells and that the intima of the entire vascular system including the 

 the valves of the heart has a like origin. 



I think that I have now shown that the endothelial fibrils 

 are constant in the heart and that we must hold the endothelial 



