274 



FRANKLIN P. MALL 



the undermining process are indicated by cross hatching. The 

 course of the atrio-ventricular muscle bundle dividing within the 

 interventricular foramen and crossing the ventricular septum, 



L.O 



/' ? * "f\ j 





R.O. 



Fig. 15 Semidiagrammatic reconstruction of the heart of an embryo 9 mm. 

 long (No. 163). The left lateral endocardial cushion has been added from anol her 

 embryo of 9 mm. (No. 422). 



A.V.B. 



Fig. 16 Semidiagrammatic reconstruction of the heart of an embryo 11 mm. 

 long (No. 353). The muscle encircling the atrial canal is reduced and the atrio- 

 ventricular bundle is seen passing behind the endocardial cushions to spread over 

 the ventricular septum below. 



is also shown. These figures indicate clearly the fate of the pri- 

 mary dividing masses of connective tissue at the base of the heart 

 in the embryo. 



