186 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



pore or point of invagination, except for a chitin plug which forms when a 

 thin chitin membrane is secreted from the outer surface of the serosa. 

 Meanwhile segmentation of the body and development of the appendages 

 and internal organs go on. The mesoderm develops as a single layer of 

 cells interrupted on the median longitudinal hne except at the cephaUc 

 end. When the dorsal ectoderm wall closes, the coelomic sacs, which had 

 remained open on the dorsal side until this time, also close. Coelomic 

 sacs are found in the antennal, gnathal, thoracic, and 10 (possibly 11) 

 abdominal segments. The sacs (Fig. 109) in structure strikingly resemble 

 those of the Blattidae. The development of muscles, fat, heart, blood 



pro.c 



ser 



Fig. 109. — Lepisma. Longitudinal section, {am. cav) Amniotic cavity. 

 (proc.) Protocephalon. (ser) Serosa, (y) Yolk. 



(mes) Mesoderm. 



cells, and nervous system resembles those of the Orthoptera. The gan- 

 glion cells of the central nervous system, however, do not cover the dorsal 

 surface of the neuropile of the ganglia. The tracheal system develops 

 rather late, the invaginations appearing about the time of the eversion of 

 the embryo from the yolk. Two pairs of thoracic and 9 (or possibly 10) 

 pairs of abdominal spiracles develop. Oenocytes seem to be lacking. 



When the segmentation of the embryo is completed the serosa con- 

 tracts anterodorsallj^, and the amnion everts, the embryo thereby being 

 drawn toward the periphery (Fig. 110). Following this the amniotic pore 

 expands, the embryo everts, and the amnion stretches over the surface 

 (Fig. HI, am) as the serosa (ser) still further contracts. The two mem- 

 branes together with the embryo thus form an envelope around the yolk. 

 The embryo now lies on the surface except for the ventrally flexed part of 

 the abdomen. The shrinkage of the serosa involves a crowding together 



