72 H. VON W. SCHULTE 



uated in the embryo of 14 somites is now interrupted and repre- 

 sented in the model of the Uimen only by a pointed projection 

 of the atrium. In the model of the endothelium, which was 

 made of this embryo, there was also a solution of continuity at 

 this point and in addition to the atrial protrusion shown by the 

 lumen there was also a small projection of collapsed endothelium 

 from the ventricle. The ends of the two processes were sepa- 

 rated by a small but perfectly definite gap. 



The fusion between the ventricles has progressed, especially 

 caudad, and the two gaps in the line of luminal coalescence are 

 tilled with epithelium. There is no longer a foramen leading 

 between the ventricles as in the 14 somite stage. At the isthmus 

 also the communication between the two tubes is greatly in- 

 creased, and from this point on the functional bulb is that of 

 the right side. The left is interrupted in its continuity and 

 represented only by irregular projection of the lumen and 

 patches of cells adherent to the wall of the right tube. 



In later stages there is no trace either of the left bulb or the 

 right atrio-ventricular canal and nothing in these slightly older 

 embryos as far as they have come under observation is indica- 

 tive of the history of these regions. The remnants of the inter- 

 ventricular partition persist awhile but are entirely effaced in 

 an embryo of 21 somites. 



The coalescence of the endothelial tubes in the cat is thus seen 

 to be delayed until the cardiac loop has been completed. It is 

 begun just prior to the initiation of the loop at points where 

 the tubes are bent mesad. In the actual coalescence angio- 

 cysts developed in relation to the middle cardiac plate are in- 

 volved. Two segments of the primitive tube are in part sac- 

 rificed — the left bulb and the right atrio-ventricular canal. 

 The factor seemingly responsible for the direction of the loop is 

 the larger size of the right omphalo-mesenteric vein with the 

 consequent displacement to the left of the region of venous 

 confluence. 



