272 FRANKLIN P. MALL 



there in the form of two cushions, the anterior and posterior endo- 

 cardial cushions. In the bulb two ridges are also formed which 

 ultimately give rise to the septum aorto pulmonale, but as this 

 latter structure has been considered by Greil and by Tandler, I 

 have taken it up only in so far as it bears upon the formation of 

 the membranous septum. 



In the embryo 3.9 mm. long (No. 463) the two endocardial cush- 

 ions of the atrial canal are well formed (fig. 4) . The posterior is short 

 and reaches from the sinus to the ventricle, while the anterior is 

 much more extensive for it reaches from the sinus also around the 

 upper and anterior part of the atrium through the atrial canal to 

 the bottom of the ventricle. In general they repeat that which 

 is shown in GreiPs fig. 3 taken from the heart of Lacerta. 18 The 

 two cushions are confined almost wholly to the left ventricle. 

 However, the side of the lower tip of the anterior cushion passes 

 through the interventricular foramen and continues as the anterior 

 medial endocardial thickening of the bulb. A little later at 4.3 

 mm. (No. 148) the same arrangement is still seen except that the 

 left lateral tip of each of the two cushions is more intimately 

 attached to the trabecular system of the ventricle. These attach- 

 ments mark the beginning of the anterior and posterior papillary 

 muscles. The cushions gradually become more and more pro- 

 nounced until the embryo is 7 mm. long (No. 2) when their lower 

 right tips begin to enter the interventricular canal. With the 

 formation of the septum primum the anterior cushion gradually 

 approaches the posterior with which it ultimately blends (fig. 9). 

 Before this takes place the septum primum is perforated forming 

 the foramen ovale II; somewhat later the foramen ovale I is com- 

 pletely obliterated. 19 By this time the two cushions have blended 

 into a solid mass which obstructs the atrial canal, leaving on either 

 side an ostium. The common cushion or valve mass is now 

 wider than it is thick (fig. 15), hangs well into the left ventricle 

 where its two corners or tips are well supported by the two papil- 

 lary muscles (figs. 11 and 12). The right half rests upon the 



18 Greil, Beitrage zur vergleich. Anat. u. Entwicklungsg. d. Herzens u. d. Trun- 

 cus arteriosus d. Wirbelthiere. Morph. Jahrbuch, Bd. 31, 1903. 



19 See also Greil, I.e., figs. 5 and 12. 



