FUSION OF CARDIAC ANLAGES IN THE CAT 59 



the part- of the i-ight. There is also axial rotation. The shoul- 

 der of the left mantle thrusts ventrad displacing the bulbils to 

 the right, and the accompanying rotation displaces the depressed 

 shoulder of the right mantle dorsad so that it is concealed in ven- 

 tral view. The loop is directed to the right, ventrad and slightly 

 caudad, and the left lateral mesocardium comes to occupy a 

 slightly more cephalic position than the right. As a whole, the 

 dorso-ventral depth of the myocardium is increased at the ex- 

 pense of its transverse breadth (fig. 15) and its extremities are 

 a little approximated, the sagittal distance between the end of 

 the bulbus and the junction of the mantles with the lateral 

 mesocardia being absolutely diminished. The dorsal meso- 

 cardium is retained unbroken in its whole extent. 



The middle plate is undergoing reduction by being absorbed 

 into the mantles. It is represented by a ridge projecting entad 

 and at the sides gradually diminishing in thickness as it fades 

 into the mantles. It can be followed into the beginning of the 

 bulb where it lies opposite the partition between the endothelial 

 tubes (fig. 14). It then runs along the convexity of the loop 

 occupying the same position relative to the fusing endocardial 

 anlages as in the bulb (fig. 15). On reaching the caudal con- 

 tour of the loop it becomes continuous with the ental eleva- 

 tion produced by the interventricular sulcus. Thus by the 

 ridge and interventricular sulcus the primitive median line of 

 the heart is represented, a conclusion which is borne out by their 

 location in their whole course opposite the line of fusion, as yet 

 incomplete, between the endothelial tubes. 



In the heart of an embryo of 16 pairs of somites the loop has 

 increased and projects more strongly. The left bulbo-ventric- 

 ular sulcus is nearly horizontal and the right has been reduced, 

 appearing only as a slight furrow ectally and a slight angle with- 

 in the myocardium. The middle plate is again represented by 

 a low ridge extending from the end of the bulb to the beginning 

 of the interventricular septum. This now has an obliquely 

 transverse direction. In an embryo of 18 to 19 pairs of somites 

 the ridge of the middle plate disappears. The interventric- 



