32 HENRY A. MURRAY, JR. 



with a right and left shoulder; d) the restricted portion between 

 ventricle and bulb, which may be called the bulbo ventricular 

 canal; e) the bulb, and /) the arterial branches. The following 

 points should be particularly noted: 1) the atrial canal has been 

 forced to the left, and that portion of the canal contributed by 

 the right heart has become relatively much reduced; 2) the left 

 shoulder of the ventricle is elevated, the right depressed; 3) the 

 left bulboventricular cleft is pronounced, the right is obscure, 

 and 4) on account of the greater impression made by the left 

 bulboventricular cleft that portion of the bulbar canal contri- 

 buted by the left heart is diminished. Later, the right bulbo- 

 ventricular cleft disappears, the left becomes more pronounced 

 and vertical, the atrium develops from the atrial canal growing 

 cephalad behind the ventricles, and the cardiac loop is then 

 complete. My observations commence at a stage when the 

 original lateral tubes have become ventrally placed and are 

 united through the intervention of a middle cardiac plate. The 

 process by which this change is effected — a subject upon which 

 Wang speculates at some length — will not be discussed. At- 

 tention will be focused on what may be considered the most 

 fundamental aspects of the succeeding modifications, namely, 

 1) the middle cardiac plate with a consideration of its future 

 history and possible connection with the interventricular septum, 

 and 2) the bulboventricular groove. 



MIDDLE CARDIAC PLATE AND INTERVENTRICULAR SEPTUM 



In figure 3 observe the middle cardiac plate connecting the 

 two hearts. It will be noticed that it is narrow cephalad, con- 

 necting the bulbs, and broad caudad between the ventricles. 

 This mesothelial element gradually becomes incorporated into 

 the myocardial walls of the enlarging ventricular cavity and is 

 later represented in the rabbit by a ridge or series of ridges mark- 

 ing the original line of fusion. The ridges which are quite ap- 

 parent on the inner aspect of the ventral wall are well shown in 

 figure 4; they are placed opposite the septa which still remain 

 between the two endothelial tubes (compare fig. 2). The early 

 embryos of the cat in the Columbia series show similar ridges 



