LATER DEVELOPMENT OF VASCULAR SYSTEM 353 



of the truncus and bulbus along the entire length of the septum, 

 and thus completely separates the aorta and pulmonary arteries 

 from each other. On the eighth day each vessel has independent 

 muscular walls, and the external constriction has made some 

 progress. 



(6) The Interventricular Septum. As noted before, the inter- 

 ventricular sulcus that extends from the bulbo-auricular angle 

 towards the apex of the heart marks the line of development of 

 the interventricular septum. The right division of the primitive 

 ventricle is therefore continuous with the bulbus and the left 

 with the atrium. However, the partition, bulbo-auricular sep- 

 tum, which at first separates the primitive right and left limbs 

 of the ventricle, undergoes rapid reduction and becomes a mere 

 ridge by the stage of ninety-six hours. Thus the opening of the 

 bulbus and the auricular canal lie side by side, separated only 

 by this slight ridge. The rotation of the ventricle brings the 

 bulbus from the right side into the mid-ventral line so that the 

 opening of the bulbus comes to lie ventral to the auricular canal 

 on its right side (cf. Figs. 199 and 200). 



In the interior of the heart the development of the inter- 

 ventricular septum is associated with the formation of the tra- 

 beculse or ramified and anastomosing processes of the myocardium 

 that convert the peripheral part of the ventricular cavity into a 

 spongy mass at an early stage. Along the line of the interven- 

 tricular sulcus these trabeculse extend farther into the cavity 

 than elsewhere, and become united together at their apices by a 

 slight thickening of the endocardium, which clothes them all, 

 thus originating the interventricular septum (Fig. 202). This 

 process begins at the apex of the ventricle, and extends towards 

 the base, the fleshy septum becoming gradually higher and thicker 

 and better organized. It thus has a concave free border, directed 

 towards the bulbo-auricular ridge and continued along both the 

 ventral and dorsal surfaces of the ventricle. The septum develops 

 more rapidly along the dorsal than the ventral wall and on the fifth 

 day reaches the neighborhood of the auricular canal on this side, 

 and unites with the right side of the fused endocardial cushions 

 which have in the meantime developed in the latter. (See below.) 

 Thus the interventricular foramen, or communication between 

 the ventricles, is gradually reduced in extent and limited to the 

 ventral anterior portion of the septum. It is never completely 



