CARDIAC LOOP IN RABBIT 



33 



and also some intermediate stages. The rabbit differs from the 

 cat in that this ental protrusion of the myocardia is a simple 

 ridge and is not surmounted by a groove. Has this ridge any 

 definite relation to the future interventricular septum? I think 

 not. After modeling a number of rabbit and cat hearts between 

 this stage an,d the stage when the interventricular septum is 

 first apparent, I find no connection between the two. As shown 

 in figure 4, when last observed the median ridge is directed to- 

 wards the atrial canal. As the latter does not change its position 

 until a later date, if the interventricular septum were a product 

 of this ridge we should expect to find it at first obliquely placed 



Fig. 3 Model of the myocardium (ventral view) from a rabbit embryo of nine 

 days; H. E. C, Ser. 620. 1, right bulb; 2, left bulb; 3, right bulboventricular 

 groove; 4, left bulboventricular groove; 5, right ventricle and shoulder; 6, left 

 ventricle and shoulder; 7, middle cardiac plate. X 100. 



and in line with the canal. This is not the case. On the con- 

 trary, we find a septum arising apparently as a thick muscular 

 ridge from the most caudal portion of the ventricular loop, cor- 

 responding to a groove on the exterior. 2 Both septum and groove 

 are sagittally placed and are not at this early stage directed to- 

 wards the atrial canal (compare fig. 6). Furthermore, the sep- 

 tum appears at a considerably later date, after the common 

 atrium is well formed and the ventricular wall has undergone 

 great expansion and growth with considerable trabecular for- 

 mation. In both the rabbit and cat embryos the increased 



2 As Mall says, it is more correct to speak of the downward growth of the 

 apices of the two ventricles than the upward growth of the septum. 



