54 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 98 



THE BODY CAVITY 



The definitive body cavity of fleas, as in other insects, is chiefly a 

 secondary one in that it is derived for the most part from the epineural 

 sinus, rather than from the lumina of the coelomic sacs. The epineural 

 sinus, in turn, has its origin as a space resulting from the reduction in 

 size of the deutoplasmic mass as the development of the embryo 

 proceeds (pi. 2, fig. 21). As the vitelline mass shrinks, it withdraws 

 from the germ band, first along the midventral region, so that the 

 resulting cavity lies immediately above the nerve cord. In its origin, 

 therefore, the cavity is specifically epineural. Later the epineural sinus 

 is extended laterally and dorsally on both sides until finally, with the 

 dorsal closure of the embryo, it entirely surrounds the mesenteron 

 which has formed in the meantime about the remaining yolk. The 

 cavities of the relatively small coelomic sacs are added to the epineural 

 sinus. The splanchnic walls of these sacs break through (pi. 10, 

 fig. 78), bringing their respective lumina into communication with 

 the extensive epineural sinus, thereby establishing the definitive body 

 cavity (pi. 2, fig. 24; pi. 11, fig. 79). 



MUSCLES 



Even in the newly hatched larva the muscles are not sufficiently 

 differentiated to facilitate identification. Therefore, no attempt was 

 made to study their individual embryological origins. It may be said, 

 however, that they have two general sources. The muscles of the 

 body wall and those which are associated with the mouthparts and 

 antennae are derived from the outer or somatic mesoderm. Some of 

 the body wall muscles are longitudinal in arrangement, whereas others 

 are oblique. In contrast to these somatic muscles, the muscles of the 

 digestive tract are derived from the splanchnic mesoderm (pi. 11, 

 fig. 80) . These are either longitudinal or circular, and their arrange- 

 ment appears to vary with the particular enteric region in question, 

 as is known to be the case in adult insects. 



FAT CELLS 



The fat cells of the flea embryo are not closely grouped to form 

 compact masses such as form the fat bodies of the older larvae. 

 Instead, they appear singly or as small irregular clumps of cells scat- 

 tered throughout the definitive body cavity. Some of them lie dorsal 

 to the dorsal diaphragm within the pericardial sinus and for this 

 reason may be called pericardial fat cells. Others which lie ventral 



