284 



FRANKLIN P. MALL 



is clear that the bundle lies just back of the endocardial cushion. 

 Coronal sections of the heart giving the position of the bundle are 

 shown in figs. 27 (46, 21 mm.), 29 (409, 16 mm.) and 30 (423, 

 15.2 mm.). In these figures it is clear that the bundle passes 

 below the medial cusp of the tricuspid valve and between it and 

 the interventricular septum. The extreme dorsal connection is 

 shown in fig. 30. In earlier stages while the interventricular fora- 

 men is still present, it is easily seen that the bundle forms its wall 

 as indicated in figs. 12 (175, 13 mm.) and 14 (424, 17.2 mm.). 



Fig. 20 Sagittal section. Embryo 15.5 mm. long (No. 390). X 18. 

 Fig. 21 Sagittal section. Embryo 18 mm. (No. 432). X 40. 



These sections show definitely that the bundle is present in the 

 youngest hearts after the atrial wall has begun to break down, and 

 that its course is exactly the same as it is in the adult. After the 

 embryo is over 20 mm. long the bundle is easily seen, as has been 

 demonstrated by other observers. Figs. 15 to 18 gives a summary 

 of what I have stated. 



The additional strips above connecting atria with ventricles 

 in various stages in embryos less than 20 mm. long may be of sig- 

 nificance in view of Romberg's and Kent's observations. Rom- 

 berg 33 found muscular connections between the anterior and pos- 



33 Krehl and Romberg, Ueber die BedeutungdesHerzmuskels, etc., Arbeiten aus 

 dermed. Klinikzu Leipzig, 1893, p. 72; and His, Jr., Die Thatigkeit des embryo- 

 nalen Herzens, Ibid., p. 25. 



