FUSION OF CARDIAC ANLAGES IN THE CAT 55 



all important respects identical. The (Mital projections which 

 thej'' occasion are shown in figure 5. 



The mid-region of the fused mantles requires some comment. 

 Here the middle-plate has been compressed to a longitudinal 

 ridge bounded b}^ well defined sulci. These are expressed entally 

 l)y ridges, which in an embryo of 13 somites extend far into the 

 bulbus. In the heart of this 12 somite embryo they are re- 

 duced in this segment and the middle region of the bulbus is 

 slightly concave. The fusion is here complete. As the bulbo- 



Fig. 5 Cephalic portion of same model as figure 4, ental view. 1, Middle 

 cardiac plate; £, fundus of right bulbo-vcntricular sulcus; 3, fundus of left bulho- 

 ventricular sulcus; 4, interior of bulbus; 5, dorsal mesocardium; 6, right ventri- 

 cle; 7, left ventricle. 



ventricidar fissures are approached the ridges begin as low ele- 

 vations which increase in height to about the middle of the ven- 

 tricular segment fading out towards the terminal constriction 

 of the heart. Between the ridges projecting from the middle 

 plate entally are occasional small processes of mesoderm, evi- 

 dently remnants of the more numerous projections of earlier 

 stages. 



As compared with the heart of the 11 somite embryo this 

 heart has lengthened somewhat, but its striking changes in con- 

 tour are associated with the deepening of the two pairs of in- 

 cisures at its lateral margin. The bulbar segment has dimin- 

 ished absolutely in breath and so in less degree has the ventric- 

 ular especially at its caudal end where it joins the lateral meso- 



