A HUMAN EMBRYO OF TWENTY-FOUR PAIRS OF SOMITES. 151 



No mention, however, is made by Mall, or by Watt 48 , who describes a similar 

 arrangement, of that portion of the atrium which projects upward and in which 

 the endothelium is not closely applied to the myocardium. 



The outer layer of the heart-tube, the so-called myo-epicardial layer, is com- 

 posed of several layers of cells, the thickness varying in different portions of the 

 heart. As Tandler 44 has shown in an embryo of 3.5 mm. (embryo Hal, Institute 

 of Anatomy, Vienna), these cells form a distinct syncytium. The myo-epicardium 

 is directly continuous at the venous and aortic ends of the heart with the trans- 

 verse septum and lining of the body-cavity, respectively, and it is by means of 

 these attachments that the heart is held in place. 



In that portion of the atrium where the myo-epicardium lies in contact with 

 the endothelium, its cells are closely applied to one another. Nuclei are crowded 

 and intercellular spaces are small. The myo-epicardium of the atrio-ventricular 

 canal is somewhat thicker than that of the atrium, and its cells are more widely 

 separated. Numerous small protoplasmic processes of the cells form a network, 

 the meshes of which are filled with delicate fibrils. In the ventricle and bulbus 

 cordis the myo-epicardium is still thicker, but in the truncus arteriosus it again 

 becomes thin. 



In describing the 3.5 mm. embryo, Tandler 44 says: 



"The myo-epicardial mantle differentiates to the extent that in the region of the 

 ventricular loop and in the bulbus its superficial layer is formed by a continuous row of 

 cells, the epicardiurn, while on the atrium and sinus, so far as the latter has a free surface, 

 no such differentiation can be said to exist." 



In my embryo I find that the outside layer of cells of the myo-epicardium 

 have not yet become flattened and detached from the underlying cells, as shown 

 by Tandler in his figure 377, but over the entire surface of the heart they are ar- 

 ranged in a distinct layer (plate 4, fig. 1). As seen in cross-section, they are in 

 places cubical and closely packed, while in other places they are more rounded and 

 farther spread apart. I am also unable to find any differentiation of the myo- 

 cardium proper into an inner spongy portion and an outer cortical portion, as 

 Tandler describes for the 3.5 mm. specimen. 



The broad space which exists between the myo-epicardial and endothehal 

 layers, according to Tandler, is filled during life with serous fluid, since it is occu- 

 pied in section by a clot-like fibrous mass which is entirely destitute of cells and 

 stains feebly with the hemotoxylin. In my specimen I find a similar clot-like 

 mass. The small, delicate fibrils form an anastomosing plexus, the meshes of 

 which are empty. For the most part these fibrils extend radially from the 

 endothelium to the myocardium, to both of which they gain attachment. They 

 radiate out particularly from the plate-like processes of the endothelium, making 

 it impossible to determine in every case where the endothelium leaves off and 

 where the fibrin begins. It seems probable that the delicate fibrils found in the 

 myocardium are due to a similar coagulation of serous fluid. 



Mitoses within the tissue of the heart are few. Occasionally one may be 

 observed in the endothelium, particularly in the cells of its plate-like processes. 



