LATER DEVELOPMENT OF VASCULAR SYSTEM 3093 
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. 
(b) 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 shght 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- 
beculee 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 trabecule extend farther into the cavity 
than elsewhere, and become united together at their apices by a 
shght thickening of the endocardium, which clothes them all, 
thus originating the interventricular septum (lig. 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 
