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cyliiidriccil (iibe in the direction of the embryonic axis, but always 

 the heart tube deviates in its course somewhat to the right (or to the 

 leftj. In longitudinal sections through the embryo the heart therefore 

 is cut obliquely, and in the same section of 4 or 5 (i we are able 

 to study the external half of the heart muscle cells (that is to say the 

 side of the cells turned towards the pericardial cavity) at the venous 

 end of the tube, the basal half of the adjoining cells, and then the 

 endocardium and the median cross section of the heart muscle cells 

 at both sides of the heai-t. 



In fig. 3 are shown the two parts of the heart nniscle cells as 

 seen in one and tlie same section. Tlie two parts of the figure are 

 in tlie section continuous, but arc lying in tUfFerent optical planes. 

 It was not possible howe\'er to reproduce the two parts in the same 

 drawing, because in the section (thickness 4 (i) different optical planes 

 ]iresented a diiferent aspect of the same point. So 1 separated the 

 two halves by a line, to indicate tlie jioinl, where the drawing is 

 made after a difTerent optical section. 



On the right side the cells of the myocard are seen from the side 

 turned towards the pericardial cavity. They appear to be separated 

 l)y distinct Itouiidaries, are very regular, and show between the cells 

 the black lines and meshes of the "Schlussleiste". On the left side 

 of the figure the basal side of the heart muscle cells is to be seen. 

 Because of the curved surface of the heart tube, at both sides the 

 cells are seen in cross section, in the median part of the tigure the 

 basal part of the cells comes into \iew\ In this part of the heart 

 muscle cells the cell membranes have completely disappeared. There 

 is only a mass of proto])lasm to be seen, which has taken a faint 

 stain ^vith eosin; imbedded in it lie thin iibrillae stained black with 

 iron-haematoxylin ; these fibrillae run for the greater part at right 

 angles to the heart axis round the heart-tube; some of them run 

 more or less obliquely (fig. 3). The same fibrilla may be followed 

 through more than one cell. At both sides the fibrillae curve round 

 and run at right angles to the surface of the section. They present 

 there a small point of a darker colour. The fibrillae are entirely 

 homogeneous. 



That these fibrillae are lying in reality only at the basal end of 

 the cell is shown in fig. 2, in which a part of the myocardium is 

 drawn as it appears in a median longitudinal section through the 

 heart tube; as the iibrillae are running here at right angles to the 

 optical plane, they appear as dots and where their course is more or 

 less oblique, as short lines. In the corners of the cells we see (at 

 the side of the cells turned towards the pericardial cavity) the black 



