386 EMBRYOLOGY OF THE LOWER VERTEBRATES oh. 



ventricle (Fig. 183, B, C, b.a.r) the ridge and its extension forming 

 the rudiment of the anterior portion of the ventricular septum. 

 The remaining and larger portion of the septum arises otherwise, 

 from a local exaggeration of the muscular trabeculae which in the 

 Bird, as in lower forms, sprout into the ventricular cavity so as to 

 convert the peripheral portion of that cavity into a sponge-work. 

 This sponge-work becomes exaggerated in the prominence and thick- 

 ness of its trabeculae along a plane marked on the external surface 

 of the heart by a distinct groove— the interventricular groove 

 (Fig. 182, C). This trabecular part of the septum (Fig. 183, C, i.v.s) is 

 at first loose and spongy but it gradually becomes condensed, at first 

 along its thickened free edge, and loses its spongy character. It 

 gradually extends forwards and becomes continuous on the one hand 

 with the bulbo-auricular portion and on the other with the septum of 

 the auricular canal. The ventricular cavity is now divided into a 

 right and a left chamber except at its anterior end where there 

 remains an interventricular foramen (Fig. 183, D, i.v.f). It will be 

 realized that but for the presence of this foramen the blood could 

 not circulate, as the only means of exit from the ventricle — the 

 opening leading into the conus — lies completely on one side (right) of 

 the original bulbo-auricular fold and, therefore, of the ventricular 

 septum of which the fold in question forms a part. As a matter of 

 fact this interventricular foramen never disappears, though it loses 

 its right to that name, for it becomes continued as a groove over the 

 surface of the mass of endocardiac tissue lying between it and the 

 conus. Eventually this groove becomes overgrown by its edges and 

 converted into a tubular channel, continuous on the one hand — 

 through the original interventricular foramen — with the cavity of 

 the left ventricle and on the other with the systemic or aortic 

 cavity of the conus. This tubular channel persists in the adult 

 condition — as the communication between left ventricle and 

 systemic aorta. 



Finally, before leaving the interventricular septum, it has to be 

 mentioned that dorsally (Fig. 183, D) it becomes continuous with the 

 bridge of endocardiac tissue which divides the atrioventricular 

 opening into a right and left half, the ventricular side of this bridge 

 growing out to meet the trabecular part of the septum. The 

 atrial side of the bridge is continuous with the atrial septum, which 

 develops here as in Lacerta, and the result is that the main part of 

 the heart is now divided into two halves, the left auricle opening 

 into the left ventricle and the right auricle opening into the right 

 ventricle (Fig. 183, E). It is to be noted however that secondary 

 perforations appear in the atrial septum (Fig. 183, E, at.s) — so as 

 to allow the systemic blood which enters the right auricle from 

 the sinus venosus to reach the left auricle, and through it the left 

 ventricle, without having to traverse the pulmonary circulation 

 during the period before the lungs are functional. 



It will be noticed in Figs. 182, C and D that in the later stages 



