DEVELOPMENT OF THE HUMAN HEART 271 



just closed, and in 432 (18) it is closed but its connection with the 

 right ventricle is still indicated. That the order of development 

 does not correspond with the length of the embryo is due to the 

 method of measuring; No. 144 was measured on the glass slide. 

 But a comparison with the profile drawings of these specimens 

 shows that the order of closure of the foramen corresponds with 

 the degree of development of the external form. In No. 423 the 

 interventricular foramen (0.1 mm. in diameter) is situated well 

 anterior, at the point of junction between the septum aorto pulmo- 

 nale and the right wing of the anterior endocardial cushion. It is 

 under the medial cusp of the tricuspid valve in exactly the position 

 taken by the atrio-ventricular bundle. In No. 424 the foramen 

 is barely 0.02 mm. in diameter, and were not the vascular system 

 injected with india ink the opening would probably be overlooked. 

 It is present in but a single section. Here it is again located with 

 the right limb of the atrio-ventricular bundle below the medial 

 cusp of the tricuspid valve well anterior. On the left side it 

 communicates with the vestibule of the aorta exactly in the posi- 

 tion the atrio-ventricular bundle lies in the adult heart. That 

 this is of significance will be considered when the atrio-ventricular 

 bundle is discussed. In this stage the posterior cusp of the aortic 

 valve still lies somewhat distant, but as the valves sink deeper 

 and deeper into the vestibule of the aorta the position of the inter- 

 ventricular opening comes to lie behind the posterior cusp adja- 

 cent to the left limb of the atrio-ventricular bundle. In an adult 

 heart with a patent interventricular foramen I have found the 

 atrio-ventricular system streaming through this opening, thus 

 showing that there is an association between them. 



The atrio-ventricular valves are not as difficult to trace in their 

 development in the successive stages as has been that of the for- 

 mation of the membranous septum. In the youngest embryos, 

 that is those under 3.5 mm. long, the endocardial connective tissue 

 which was quite equally distributed in the earliest stages has 

 gradually rearranged itself . first becoming less pronounced in the 

 atria and then becoming well dove-tailed with the trabecular 

 system in the ventricle and bulb. As soon as the atrial canal 

 is well formed the endocardial connective tissue arranges itself 



