DEVELOPMENT. 



647 



chyme cells on the other. The most dorsal edge of the 

 somewhat triangular coelomic pouches gives origin to a row of 

 cardioblasts or cells destined to form the walls of the heart. The 

 pericardium arises a little later, also from the mesodermic 

 cells. 



FIG. 407. Transverse sections through the abdominal region of two consecutive stages in 

 development of Phyllodromia germanica (after Heymons). am amnion ; bg rudiment of 

 the ventral chain of ganglia ; c coelomic cavity ; c' dorsal, and c" ventral portion of the 

 coelomic cavity ; cz cells of the primitive segments which become applied to the genital 

 rudiment ; d food-yolk ; dw dorsal wall of the coelomic sac ; ec ectoderm ; ep epithelial 

 cells ; ex abdominal limb-rudiments ; / rudiment of the fat-body ; gz genital cells ; Iw 

 lateral wall of the coelomic sac ; m mesoderm-cells which do not take part in the formation 

 of the coelomic sacs ; mw median wall of the coelomic sac ; vm ventral longitudinal muscle - 



The muscles of the Insect body are differentiated from the- 

 cells of the somatic mesoderm, from which the fat body also 

 originates. 



The cells which form the reproductive elements are recogniz- 

 able at an early stage, indeed, in the Diptera and Aphidae, before 

 the blastoderm is definitely established. In other Insects they 

 are conspicuous by their large size lying in the splanchnic wall 

 of the coelomic sacs from the second to the seventh abdominal 



