34 HENRY A. MURRAY, JR. 



growth in thickness of the ventricular wall is first manifest on 

 the ventral wall and gradually spreads laterally and dorsally. 

 In my series, the actual height 3 of the heart in the rabbit when 

 the last sign of a middle cardiac plate can be determined is 0.32 

 mm., whereas the earliest sign of an interventricular septum is 

 found in an embryo with an actual heart length of approximately 

 0.75 mm. Doctor Schulte, in the latest stage in which a middle 

 cardiac plate was present in his material, found it continued 

 caudad by a sulcus which he regarded as the beginning septum 

 ventriculorum. Beyond this stage there was a gap in his ma- 

 terial to the period of a well-developed septum with no remnant 

 of the middle cardiac plate. My reason for dissenting from his 

 conclusion that the median plate gives rise to the septum is that 

 in the rabbit models and in embryos of the cat subsequently 

 obtained the sulcus mentioned is found to disappear and the 

 ventricle becomes evenly convex without a trace of indentation 

 referable to the middle cardiac plate. Only later does the ventri- 

 cular septum arise in the manner I have described. The interior 

 of the model portrayed in figure 5 shows a smooth wall with no 

 sign of a ridge, although at two points the endothelial tubes have 

 not yet completely united. Considerably later, as the atrial 

 canal is moved to the right while the interventricular septum 

 remains fixed, the canal comes to be immediately dorsal to the 

 septum ; the septum will then be seen to extend toward the centre 

 of the canal and still later to become fused to the endocardial 

 cushion. The subsequent development of these parts is beyond 

 the scope of this paper. 



THE BULBOVENTRICULAR GROOVES 



These clefts first appear on each side before the lateral cardiac 

 vessels have fused. They have been given this name because 

 in their primary position they separate primitive bulb from 

 primitive ventricle. Later, as we shall see, the bulb contributes 

 to the right ventricle, and the left bulboventricular cleft may 

 then be termed an 'interventricular groove.' In figure 3 the 



3 As measured from the most cephalic to the most caudal points, regardless 

 of what portions of the heart these may be. 



