262 FRANKLIN P. MALL 



In an embryo 4.3 mm. long (No. 148) practically the same con- 

 ditions are seen as in the embryo just described. If anything 

 it is a little more advanced in development. A section of the 

 endocardial cushion is shown in fig. 5. From now on new con- 

 ditions arise which when concluded separate the heart into its 

 right and left halves. 



The anterior and posterior cushions are now well formed, the 

 superior septum (primum) and the septum of the ventricles (sep- 

 tum inferior) are beginning but the septum aorto-pulmonale 

 (aortic septum) is still absent. While these are forming, up to 

 the next stage, when the muscle wall of the atrial canal begins to 

 break down, the heart gradually enlarges without changing very 

 markedly its external form. The steps which I am about to de- 

 scribe are well established in various mammals, but I shall repeat 

 them hastily in order to confirm them all in the human heart. In 

 doing this I shall include in the descriptions the following speci- 

 mens : 



Embryo No. 80, C. R. length 5 mm. 



Embryo No. 136, C. R. length 4 mm. 



Embryo No. 116, C. R. length 5 mm. 



Embryo No. 241, C. R. length 6 mm. 



Embryo No. 2, C. R. length 7 mm. 



Embryo No. 383, C. R. length 7 mm. 



Embryo No. 380, C. R. length 7.5 mm. 



Embryo No. 113, C. R. length 8 mm. 



Embryo No. 397, C. R. length 8 mm. 



Embryo No. 422, C. R. length 9 mm. 



Embryo No. 163, C. R. length 9 mm. 



In Embryo No. 80 the anterior and posterior cushions are con- 

 siderably thicker than before but hold practically the same rela- 

 tion to the heart as in Nos. 463 and 148. In the left ventricle the 

 cushions are spread out and have attached themselves to the 

 trabecular system. As there are two attachments which corre- 

 spond in position to the anterior and posterior papillary muscles, 

 it is proper to speak of them as such. The septum aorto-pul- 



