NO. 3 EMBRYOLOGY OF FLEAS KESSEL 55 



to this diaphragm and within the perivisceral sinus, are the perivisceral 

 fat cells. No fat cells were observed in the perineural sinus. 



The embryonic fat cells of fleas are very similar to the adipose cells 

 of the higher animals. They appear to become distended with oil at 

 a very early age. As in the adipose cells of mammals, the nucleus is 

 displaced to the surface where it appears like the set in a ring. This 

 condition differs from the observations of Nelson (191 5) on the newly 

 hatched larva of the honeybee in which form the fat cells only occa- 

 sionally were found to possess a minute fat globule. 



The embryonic origin of the irregularly placed fat cells in the flea 

 embryo is difficult to follow in detail. They are definitely mesodermal 

 derivitives, however. From their positions, it appears that the peri- 

 cardial fat cells come from the somatic layer, whereas the peri- 

 visceral ones are derived from the splanchnic mesoderm adjacent to 

 the developing enteric muscles. 



CIRCULATORY SYSTEM 



The circulatory system of the flea larva is composed of the dorsal 

 blood vessel (heart) and the various sinuses of the haemocoel. The 

 dorsal blood vessel is one of the last structures to be formed. It is 

 derived, as in other insect embryos, from cells which are known as 

 cardioblasts. At the time of their first appearance, they constitute a 

 pair of narrow longitudinal bands, one band located dorsolaterally on 

 each side of the body along the line of junction between the somatic 

 and splanchnic portions of the mesoderm. With the dorsal growth 

 of the mesoderm and the corresponding extension of the body cavity 

 as the vitelline mass shrinks away from the upper surface of the 

 egg, the cardioblastic bands gradually approach one another and 

 ultimately meet along the dorsal midline. The apposition of the 

 splanchnic mesoderm to the surface of the already completed mid- 

 intestine epithelium is simultaneous with this process. Some time 

 before the two heart-forming bands of cells unite, these become 

 separated from the enteric muscle-producing portion of the meso- 

 derm. The cardioblasts never lose their connection with the somatic 

 mesoderm, however. The actual union of the two cardioblastic bands, 

 to form the dorsal blood vessel, occurs shortly afterward. This tube 

 extends well into the head region. 



The open portion of the circulatory system in the flea larva is 

 similar to that of other forms. It consists of the definitive haemocoel 

 which is formed from the completed and partially partitioned epi- 

 neural sinus plus the coelomic sacs. This partitioning takes place by 



