FUSION OF CARDIAC ANLAGES IN THE CAT 63 



of angiocysts. The tliircl is well designated as that of the endo- 

 thehal tube with a continuous lumen but in this stage there may 

 also be separate angiocysts and mesenchyme adjacent to the 

 tube. 



The heart of the embryo of 9 pairs of somites is an ex- 

 ample of this last mentioned state. In it the endothelial tube 

 on each side has a continuous lumen not of equal diameter 

 throughout, it is true, for it has t\\() constrictions and its walls 

 still show traces of their component angiocysts. The con- 

 strictions are located at the bulbo-ventricular sulcus and at the 

 cardio-venous angle. The former reduces the lumen to a nar- 

 row dorso-^'entral cleft, the latter produces a smaller but quite 

 perceptible diminution of the lumen. The bulb and ventricle 

 are on the contrary dilated, and markedly so in their transverse 

 diameter. 



Between the bulbs corresponding to the narrow middle cardiac 

 plate there is room for but little mesenchyme. Between the 

 divergent ventricles, however, it is more abundant and stretches 

 across between the endothelial tubes in plates and anastomosing 

 strands, among which are scattered small angiocysts (fig. 2). 



Conditions in the embryo of 11 somites (figs. 9 and 10) differ 

 only in that the tubes are approximated and nearly parallel in 

 their whole length. The slight asymmetry of the two sides is 

 due mainly to the collapse of the shoulder of the left ventricle 

 and in the cast of the lumen to a similar collapse of the right 

 omphalo-mesenteric vein, which in other embryos of about this 

 stage is actually a little larger than the left. 



The bulbs are dilated and merge into the ventral aortic roots 

 and the plexus forming about the foregut. As yet fusion has 

 taken place across the median line only at one point situated well 

 forward towards the oral plate. Elsewhere between the bulbs 

 are strands of mesenchyme in which are two small angiocysts. 



Corresponding to the bulbo-ventricular sulcus on each side is 

 a narrow isthmus compressed laterally. 



The ventricles are wide, prolonged into the shoulders of the 

 mantles and diminishing caudad to their junction with the 

 oinphalo-mesenteric veins. The right in four places shows rem- 



