FIG. 69. Diagrams of stages in the development of the Earthworm. A, blast ula (surrounded 

 by a membrane) ; B, section of a blastula showing blastocoel and one of the primary cells (pole 

 oells) of the mesoderm; C, later blastula with developing mesoderm bands; D, start of gastrula- 

 tion; E, lateral view of gastrula showing invagination, which as it proceeds leaves the mesoderm 

 bands on either side of the body as indicated by the cells represented with dotted outline; 



F, section of E, along the line S-S to show pole cells, mesoderm bands, and enteric cavity. 



G, later stage showing cavities in the mesoderm bands, H, the same (G) in cross section; 7, dia- 

 gram of a longitudinal section of a young worm after formation of mouth and anus ; J, the same in 

 cross section; K, later stage in cross section, al, alimentary canal; on, anus; or, enteric cavity; 

 coe, coelom; ec, ectoderm; en, endoderm; m, primary mesoderm cells or pole cells; m?, mesoderm; 

 mh, mouth; n, nerve cord; , cavity of segment; sc, blastocoel; sm, somatic layer of meso- 

 derm which with the ectoderm forms the body wall; splm, splanchnic layer of mesoderm which 

 with the endoderm forms the wall of the alimentary canal, (From Sedgwick and Wilson.) 



126 



