160 



ORGANS AND ORGAN SYSTEMS 



C 



splrn. 



mh 



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, al, alimentary tract; an, anus; ar, 

 archenteron; c&, coelom; ec, ectoderm; en, endoderm; m t mesoblast pole-cell; 

 m z , mesoblast tissue; mh, mouth; n, nerve cord; sc, segmentation cavity; sm, 

 somatic mesoderm; spl.m, splanchnic mesoderm; 8, eighth coelomic cavity. 

 (From Sedgwick and Wilson.) 



