76 EDWAED MCCRADY, JR. 



is also still proliferating from the edges of the posterior end 

 of the neural plate. 



Fusion of the hearts in the bulbar region. The process 

 which leads to the fusion of the paired lateral hemicardia into 

 the single median heart began when the pericardial sacs, 

 which were completely separate in stage 23, grew forward 

 around the head region and met in the midline in stage 24 

 (fig. 20, totomount, I and J). This region of first fusion of the 

 pericardial sacs is anterior to both the aortic and cardiac 

 primordia. The aortic plexuses lie for the most part out- 

 side the pericardia, but, as was shown in figure 20, J, the 

 point of common origin of the first and second arches, and 

 even a portion of each of these arches, is at first covered by 



acovsticoTacicil 



Fig. 24 Lateral view of reconstruction of stage 25 (17137). 



myocardium. Accordingly, as the' region of fusion of the 

 pericardia progresses caudally the first portion of the endo- 

 cardia to fuse is this bulbar portion. And the endothelial 

 tubes which form the bases of the first aortic arches actually 

 fuse before any part of the hearts proper. This is the con- 

 dition in the last specimen of stage 24 (17139). In stage 25 

 (fig. 25, reconstruction and sections E and F) the fusion has 

 not progressed beyond the bulbus cordis, but the ventricular 

 portions of the tubes have been brought into close apposition. 

 The reconstruction figure shows also that the atria are on 

 either side of the anterior intestinal portal and are still widely 

 separate. 



The process of apposition and fusion of the pericardia and 

 heart tubes in the opossum is somewhat peculiar in that it 



