282 FRANKLIN P. MALL 



rather inclined to think that it is a new formation, as has been 

 asserted by Retzer. 29 



In following the atrio-ventricular bundle in the foetus Fahr 30 

 found it in one of 160 mm. long, Tawara 31 in one 100 mm. long, 

 Monckeberg 32 at 75 mm., Tandler in human embryos 28.5 and 19 

 mm. long, and Retzer in the pig from 15 to 20 mm. long. His, 

 Sr., states that the muscle wall between the atria and ventricles 

 is nearly broken down in an embryo 13.8 (?) mm. long. It is 

 evident that if the bundle is a new formation it must appear in 

 embryos about 15 mm. long. However, in order to anticipate my 

 own report upon this subject it may be stated that the bundle is 

 not a new formation but is the remnant of the wall of the atrial 

 canal after its anterior and lateral sides have been broken in the 

 formation of the tricuspid and mitral valves. The portion on 

 the posterior wall which connects the sinus with the ventricle never 

 breaks down; early in development it shows changes in structure 

 which differentiate it from the rest of the heart muscle. It holds 

 a constant position just behind the posterior endocardial cushion 

 immediately over the septum of the ventricle, a position which 

 is marked at first by the interventricular foramen and later by the 

 membranous septum. 



The atrial canal is first well formed in an embryo 3.9 mm. 

 long (No. 463); it is now markedly constricted and is pretty well 

 filled up by the two large endocardial cushions (fig. 4) . Its 

 outer wall is uniformly muscular throughout. There is practically 

 no changes in subsequent stages until the lateral endocardial 

 cushions make their appearance (fig. 15), both of which are pres- 

 ent in an embryo 11 mm. long (No. 353, fig. 16). At this time 

 there is a break in the region of the left cushion, and possibly over 

 the right cushion, as well as in the bottom of the bulbo-ventricular 

 groove, that is between the aorta and the left ventricle. At 

 13 mm. (No. 406) the break is more marked over the left endocar- 



29 Retzer, Anat. Rec, vol. 2, 1908. 



30 Fahr, Virchow's Archiv, Bd. 188, 1907. 



31 Tawara, Das Reizleitungssystein des Saugethierherzens, Jena, 1906. 



32 Monckeberg, Untersuchungen liber des Atrioventriklarbiindel in mensch- 

 lichen Herzens. Jena, 1908. 



