490 



the abdomen; it alone is contractile, and is not surrounded 

 by pericardium. Its walls consist of two layers, an inner 

 exceedingly fine and delicate "endothelium," the transformed 

 larval heart; and an ou.ter layer of broad striated muscle 

 fibres, circularly disposed, corresponding and homologous with 

 the pericardium. The aorta, which is continuous with it in 

 front, gradually bends down, and lies just dorsal to the 

 oesophagus (cf. fig. 154). It is, of course, composed of the 

 same two layers (fig. 219). 



During larval life a few large rounded cells lie in close 

 connection with the heart. They resemble the cells of the 

 dorsal abdominal glands in appearance. So far as I could 

 observe, they disappear late in pupal life. 



As the thoracic intestine, some six hours after pupation, 

 begins to assume its almost insignificant proportions, the 

 anterior part of the heart (aorta) sags downwards, and comes 

 to lie just dorsal to the oesophagus, i.e., a little below the 

 mid-region of the thorax. 



I have not been able to observe the heart-beat during 

 metamorphosis, the heart being too obscured by the surround- 

 ing fat-body. In the head of the pupa, however, movements 

 of the fat-body regularly occur, and this is due, doubtless, 

 to the beating of the heart. 



Appendix. 



THE DEGENERATION PROCESSES OF THE 

 LARVAL CELLS OF NASONIA. 



The physiological interpretation which we place on the 

 insect transformation depends in the main on our knowledge 

 of the condition of the larval cells at the time of meta- 

 morphosis. Do the cells die, and is it only dead material that 

 the leucocytes absorb; or are they attacked by the leucocytes 

 while still alive? It is the former opinion, to a certain extent, 

 that Berlese holds: ''Phagocytosis never occurs, and amoebo- 

 cytes only become active when the muscle has disintegrated 

 through internal causes." His amoebocytes were, mainly, at 

 any rate, embryonic cells — "Myocytes," "Splanchnocytes," 

 etc. — but there can be little doubt that his "sarcolytes" were 

 really gorged leucocytes, and that phagocytosis does occur is 

 certain. But is it phagocytosis of dead or living larval cells? 



Perez has written in favour of the latter view. "I think 

 I have proved satisfactorily that there is no spontaneous 

 fragmentation of this organ into sarcolytes, as Berlese 

 thought." He observed the phagocytes entering muscles 



