36 HENRY A. MURRAY, JR. 



grooves are horizontal. The left groove is already assuming a 

 more significant r61e and the asymmetry forecasts future changes. 

 In the cat this prominence of the left cleft is not accentuated until 

 somewhat later. A subsequent stage in the rabbit is represented 

 in the endothelial model, figure 2, and in figure 5 a fully developed 

 cardiac loop is seen. The bulboventricular cleft (there is now 

 only one, the right having become obliterated) is more oblique. 

 From this stage onward there is a continual progressive ex- 

 tension of this furrow, and as it develops, its plane is modified 

 so that in figure 6 we find it very nearly vertical. Notice in 

 this drawing its relationship to the primitive septum. It is not 

 in line with the latter structure. In the next period, however, 

 the bulboventricular groove, formerly horizontal, is now vertical, 

 protrudes into the ventricular chamber, becomes continuous 

 with the septum, and together with it divides the cavity into 

 right and left portions. This is well shown in figure 7. The 

 division of the common ventricle then seems to be the result of 

 four processes: 1) the interventricular septum growing cephalad 

 from the floor of the loop; 2) the bulboventricular groove becom- 

 ing vertical and forming the ventral portion of the septum; 3) 

 the migration of the atrial canal to the right, allowing the endo- 

 thelial cushions to play their part, and finally, as His has shown, 

 4) the downgrowth of the pulmonoaortic septum which fuses with 

 the above-mentioned elements so as to form a continuous par- 

 tition between the right and left hearts. 



Fig. 6 Model of the myocardium (dorsal view of the ventral wall) from a rabbit 

 embryo of ten and one-half days; H. E. C, Ser. 559. 1, bulb; 2, ridge extending 

 into the common ventricular cavity and corresponding to the bulboventricular 

 cleft; 3, shoulder; 4, interventricular septum (this is the first indication of the 

 ridge found at the apex of the ventricular loop). X 100. 



Fig. 7 Model of the endocardial cavity (ventral view) from a cat embryo of 

 7 mm. ; Columbia Collection, Series 266. By kind permission of Dr. A. J. Brown. 

 In the Harvard Laboratory there is a very similar model of the cavities in the 

 heart of a 4.4-mm. pig embryo, made in 1909, under Dr. Minot's direction, by Mr. 

 A. E. Meyers. 1, cleft made by the ridge growing upward from the caudal 

 extremity of the loop, which is continuous with 2, the impression made by the 

 bulboventricular groove. Together they partially subdivide the ventricular 

 cavity; 3, atrial canal; 4, left atrium. X 50. 



