FURTHER DEVELOPMENT OF THE MESODERM. 



319 



/ 



I 



Fig. 157. — Transverse sections through the abdominal region of three consecutive stages in 

 development of PhyUod/romia germanica (after Heymons). am, amnion ; [7«/, 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; <', food-yolk; dw, dorsal wall of the coelomic sac; ec, ectoderm; ep, epithelial 

 cells; <?./-, abdominal limb-rudiments; /, rudiment of the fat-body ;~ js.Tgenital cells; 

 Iw, lateral wall of the coelomic sac; m, mesoderm-cells which do n<>t]take part in the 

 formation of the coelomic sacs : mm, median wall of the coelomic sac ; so, somatic mesoderm- 

 layer ; vm, ventral longitudinal muscle. 



