FORMATION OF THE BODY-CAVITY 



565 



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

 the primitive segments are not so extensively developed (Fig. 539, D-F , us). 

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

 primitive baud which correspond to the dorsal section of the coaloin-sacs of 



a/m. 



Fir,. 540. Cross-sections through the abdominal part of throe successive stages of evolution 

 of Phyllpdromia </< ////. miai : <nn, uninion ; />(/, rudiment of the ventral nervous chord : c. co-lo- 

 tnic cavity; c', dorsal, and c". ventral, section of the ciplomic sac; cz, cells of the walls of the 

 primitive segment, which are joined to the genital rudiments; ;/z, genital cells; d><\ dorsal wall 

 of the co'lomic sac ; <l. yolk : cc, ectoderm ; ep, epithelium-cells ; <., rudiment of the abdominal 

 appendages ; /", t r erm of the lat-hody ; !>r, lateral wall of the co'lomic sac ; 'in, mesoderm cells, which 

 take no part in the formation of the co-lomic sac; mic. median wall of the co-lomic sac; xo, somatic 

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



