156 



ORGANS AND ORGAN SYSTEMS 



FIG. 66. Diagrammatic figures of the early and embryonic stages in develop- 

 ment of the earthworm. A-F, gastrulation showing the formation of the two 

 layers ectoderm and endoderm, and the beginning of the mesoderm arising at the 

 outset from the cells shown at m\ the lower five figures show the origin of the 

 coelomic chambers and the dissepiments. (From Sedgwick and Wilson.) 



