FXTSION OF ('AUl)IA(^ ANLAGES IN THE CAT 71 



iiig ventrjiil -is well as ccpluihitl teiuls to tlirow the left mantle 

 ventrad and so initiates the axial rotation begun in this stage. 

 These changes of position on the part of the left heart would 

 seem to entail as consequences the observed displacements of 

 the right, the opening out of the right bulbo-ventricular sulcus, 

 the reduction of the shoulder of the right mantle, and its 

 rotation dorsad. 



But there is an additional factor to be considered. The ac- 

 centuation of the left bulbo-ventricular sulcus increases the com- 

 pression of the isthmus of that side and in so far impedes cir- 

 culation through it. This condition favors the engorgement of 

 the left ventricle and so participates in producing the effects 

 enumerated above. However, prior to these events a connec- 

 tion has been formed between the two isthmi close to their junc- 

 tion with the ventricles. When the left isthmus is compressed 

 this communication serves as a collateral channel leading the 

 blood stream into the right bulb which from now on exceeds the 

 left in development. The distension of the right ventricle is 

 maintained by the interventricular communication already 

 described. There is need of some arrangement of this sort for 

 the communication of the atrium with the right ventricle is of 

 smaller caliber than on the left side. This also depends upon 

 the shift of the venous end of the heart to the left with a conse- 

 quent marked accentuation of the right cardio-venous angle 

 and a diminution of angularity on the left. 



The excess of the right omphalo-mesenteric vein over the left 

 seems then the efficient cause of the displacement of the atrium 

 to the left and this joined with the configuration of the tubes 

 and mantles at the beginning of the process is capable of afford- 

 ing a mechanical explanation of the formation and direction of 

 the loop. 



Conditions in the embryo of 16 pairs of somites are corrobo- 

 rative of the findings in the embryo just described. The lumen 

 of the heart is shown in figure 16. The right omphalo-mesen- 

 teric is the larger, the atrium is strongly displaced to the left and 

 joins the atrio-ventricular canal almost at right angles. Here 

 an important change has been effected for the right tube, atten- 



