360 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



and divides into metameres. In most of the somites formed by this 

 process, there is developed a pair of coelomic cavities, each cavity- 

 bounded by thick walls, similar to that described by Heider (1889) for 

 Hydrophilus. These sacs do not communicate with each other. The 

 number of pairs occurring in the fleas appears to be 16, the most anterior 

 lying in the deutocerebral segment. There is no indication of their 

 presence in the preantennal or in the intercalary segment. Of the 

 remaining pairs occurring in the flea they are located in the first 15 seg- 

 ments posterior to the stomodaeal invagination. 



The definitive body cavity of fleas is chiefly a secondary one in that it 

 is derived for the most part from the epineural sinus, as in other insects, 

 rather than from the cavities of the coelomic sacs. As the yolk reduces 

 in size, a space is left constituting the epineural sinus. As the yolk 

 shrinks, it withdraws from the germ band, first along the mid-ventral 

 region, so that the resulting cavity lies immediately above the nerve 

 cord. Later, the epineural sinus extends laterally and dorsally on both 

 sides until finally, with the dorsal closure of the embryo, it entirely sur- 

 rounds 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, bringing their respective lumina into communication with the 

 extensive epineural sinus, thereby establishing the definitive body cavity. 



Of the other mesodermal structures, the development of the muscles, 

 fat, haemocytes, and circulatory system offers little that differs from that 

 of other insects. From their positions, it appears that the pericardial 

 fat cells come from the somatic layer, whereas the perivisceral ones are 

 derived from the splanchnic mesoderm adjacent to the developing enteric 

 muscles. The gonads are prominent embryonic organs. They are pro- 

 duced by the formation of a splanchnic mesoderm sheath about the 

 germ cells which have separated into two groups and migrated laterally 

 and anteriorly during the development of tlie embryo and come to lie 

 in abdominal segments four to six. 



The invaginations of the tracheal system make their first appearance 

 about the time the coelomic sacs arise and while the neural groove is still 

 open. The invaginations themselves deepen, branch, and anastomose to 

 form the complex respiratory system of the larva. The definitive number 

 of spiracles found in the fully developed embryo is 10 pairs. All of them 

 arise in their definitive position except the first which originates on the 

 mesothorax and migrates forward to its larval position during embryonic 

 development. The oenocytes arise from the lateral ectoderm of the 

 anterior region of the abdomen, and metastigmatic invaginations are 

 differentiated. At first resembling other ectodermal cells, they soon 

 increase in size and assume their characteristic appearance. 



