THE EARTHWORM 



217 



illustrates the development of the nervous system out of 

 the ectoderm. The organs and tissues of the earthworm which 

 originate from the germ-layers may be stated briefly as follows. 

 From the ectoderm come the outer skin and the nervous 

 system ; from the mesoderm develop the muscles, the blood- 

 vessels, the reproductive organs, arid probably the nephridia ; 

 from the endoderm comes the lining of the alimentary canal. 



ec 



an 



FIG. 106. Late Stages of Development of Earthworm. Much enlarged 



A, longitudinal section of gastrula, beginning of portions of body-cavity ; 7>, trans- 

 verse section of gastrula, same stage as A ; C, transverse section of gastrula, 

 body-cavity enlarging, mesoderm divided ; D, transverse section, inner por- 

 tion of mesoderm lying against endoderm, outer portion against ectoderm 

 E, longitudinal section of young earthworm, alimentary canal fully formed 

 ar, archenteron ; an, anus ; al, alimentary canal ; ec, ectoderm ; en, entoderm 

 coe, body-cavity (coelom) ; m, pole-cells of mesoderm ; m 2 , mesoderm 

 mh, mouth; n, ventral nerve-chain; s, cavity of somite; s.m, outer layer of 

 mesoderm ; spl. in, inner layer of mesoderm 



(From Sedgwick and Wilson's General Biology} 



