FORMATION OF THE BODY-CAVITY 



565 



In the highest groups of insects (Coleoptera, Lepidoptera, and Hymenoptera) 

 the primitive segments are not so extensively developed (Fig. 639, D-F, us). 

 They here form only relatively small sacs situated in the lateral parts of the 

 primitive band which correspond to the dorsal section of the coalom-sacs of 



FIG. 540. Cross-sections through the abdominal part of three successive stages of evolution 

 of Phyllodromia germanica : am, amnion ; by, rudiment of the ventral nervous chord ; o, ccelo- 

 mic cavity; c', dorsal, and c", ventral, section of the cnelomic sac; cz, cells of the walls of the 

 primitive segment, which are joined to the genital rudiments ; ffs, genital cells ; dip, dorsal wall 

 of the coploinic sac ; d, yolk ; eo, ectoderm ; ep, epithelium-cells ; ex, rudiment of the abdominal 

 appendages ; /, germ of the fat-body ; he, lateral wall of the ccelomic sac ; in, mesoderm cells, which 

 take no part in the formation of the ccelomic sac ; into, median wall of the ccelomic sac ; no, somatic 

 inesoderm layer ; vm, ventral longitudinal muscle. After Heymons, from Korschelt and Heider. 



