290 FRANKLIN P. MALL 



describes and pictures it in a specimen of the fifth month. Fahr 

 states that the bundle is less pronounced in the foetus than in the 

 adult but that its position is the same. Tandler found it in an 

 embryo 20 mm. long as "distinguishable even under low powers 

 by its staining properties." The nuclei are dark and the cell 

 border stains faintly with eosin. At 20.5 mm. the cells of the 

 bundle have become larger both in the atrium and the ventricle. 



In suitable sagittal sections of the heart the atrio-ventricular 

 bundle may be recognized in an embryo 8 mm. long (fig. 9) as 

 a strand of tissue extending from the region of the sinus to the 

 ventricle. This tissue is somewhat separated from the rest by 

 delicate spaces and its cells stain somewhat more intensely than 

 the rest. In embryo pigs 7 mm. long the bundle appears more 

 like epithelium with a tendency towards vesicle formation in the 

 immediate region of the sinus. In larger foetuses of the sheep the 

 epithelial nature is most pronounced. In an embryo 11 mm. 

 long (figs. 24 and 25) the numerous small spaces around the bundle 

 encircle it from the region of the sinus just behind the posterior 

 endocardial cushion, along its course behind the medial cusp of 

 the tricuspid valve to the upper border of the ventricular septum. 

 This same arrangement is found in another embryo (12.5 mm. long, 

 fig. 6) cut in the same plane and stained in the same way. In 

 this specimen the muscle fibrils are not present in the bundle but 

 they are found in the muscle cells of the atria and the ventricles. 

 In a sagittal section of an embryo 14 mm. long (fig. 13) the bundle 

 is recognizable throughout its extent by its surrounding spaces 

 and not by its staining properties. In 175 (fig. 12) the bundle 

 can be followed into the two ventricles. The same structure and 

 arrangement is made out in embryos 15.2 mm. (423), 15.5 mm. 

 (390), 16 mm. (409, fig. 29) and 17.2 mm. (424., fig. 14). In 

 older specimens the interventricular foramen is just closed and 

 the bundle reaches to it from the region of the sinus. Two limbs 

 are easily outlined one of which reaches to the moderator band of 

 the right ventricle, which is pronounced in the human embryo. 



The atrio-ventricular muscle is well lodged between the valve 

 and the annulus fibrosus in an embryo 18 mm. long (432, fig. 21); 

 it again extends into the moderator band. The same is observed 



