489 



34/x in length. The nucleus is rounded or oval, measuring 

 5/x by 12fx, and has a small nucleolus. 



The Metamorphosis of the Heart. 



It is at about the time of defaecation that the heart 

 begins its transformation, and the metamorphosis is very pro- 

 found. In the period just prior to defaecation the pericardial 

 and heart cells begin to undergo a granular degeneration. The 

 nuclei show the usual great nucleoli, and the cells the usual 

 hypertrophy (fig. 214). At this time it becomes possible to 

 distinguish clearly the larval from the imaginal elements 

 within the heart tissues (fig. 213). The imaginal heart, how- 

 ever, is regenerated not only from scattered embryonic cells 

 within its_ walls, but also from a mass of embryonic cells lying 

 just below the heart, and dorsal to the rear of the midgut. 

 I have not been able to locate this structure in the early 

 larva, but at the time of defaecation a column of these 

 embryonic cells may be observed extending upwards and along 

 the ventral side of the pericardium. Proliferation is very 

 rapid and the cells advance quickly, absorbing the larval 

 elements as they grow (figs. 214, 215, 216). The heart itself 

 is rejuvenated mainly or entirely from the imaginal cells 

 within its walls ; only the pericardium seems to arise from the 

 "sub-pericardial imaginal disc." In the larva eight hours 

 after defaecation the heart tube has been completely rejuven- 

 ated, and below it, and in close contact with it, lies a long 

 band of cells, the renovated "pericardium" (fig. 217). The 

 nuclei of the (true) heart cells are large and bulge into the 

 lumen of the tube (figs. 217, 219). At this period the 

 imaginal pericardial cells are in process of rapid division. 

 They quickly grow upwards (fig. 218) and soon form another 

 tube on the outside of, and in close contact with, the 

 renovated heart. The pericardium, therefore, no longer 

 forms a loose sac on the outside of the heart, but actually 

 becomes a part of it (fig. 219). In the region of the stomach 

 the ''compound" heart remains as a rather wide sac-like tube. 

 Ostia become developed, but I have not observed their dis- 

 position accurately. In this region, also, the "pericardial" 

 cells, which have formed a membrane in close contact with 

 the true heart tube, transform themselves into striated muscle 

 cells (fig. 220). This part of the heart alone is contractile. 

 All the more anterior part (i.e., in the thoracic and anterior 

 abdominal regions) is to be looked upon as an "aorta." It is 

 composed, of course, of the ordinary heart tube, and the 

 surrounding closely fitting renovated pericardium (fig. 219). 



The dorsal vessel of the adult wasp consists, then, of a 

 short contractile chamber lying in the posterodorsal region of 



