EMBRYOLOGY OF THE EARTHWORM 



157 



known as nephroblasts . All give rise to sheets of cells which be- 

 come differentiated into the ultimate adult structures nervous 

 system, muscles and nephridia. 



Meanwhile the mass of mesoderm cells on each side of the 

 median line begin to show traces of a loose structure, and later 

 well-marked spaces or cavities are developed from these spaces 

 and assume regular shapes. They are first clearly formed in the 

 region of the bi^stopore as regularly arranged cavities lined by 

 mesoderm cells. These cavities are the coelomic cavities of the 

 adult and their anterior and posterior walls form the dissepi- 

 ments marking out the metameres of the mature animal. Thus 

 the coelomic cavities are mesodermal in origin and are lined by 

 mesoblast, this lining, known as endpthelium, thus having a 

 different origin from the epithelium of the gut which comes from 

 the primary endoderm or from the epithelium of the skin which 

 comes from the primary ectoderm. The primitive enteric 

 cavity of the gastrula gradually develops through a series of 

 changes into oesophagus, crop, gizzard, and stomach intestine. 

 The ectoderm turns in at the mouth end and at the posterior 

 end where the anus breaks through. Two regions of the diges- 

 tive tract are thus lined by ectoderm, that of the mouth, termed 

 the stomodaeum, and that of the anal end called the proctodaeum. 

 The chlorogogue cells are formed from mesoderm as are also 

 the blood vessels, muscles, reproductive organs and seta sacs. 

 The young worm is now ready for an independent life and it 

 leaves the coccoon after from two to three weeks. 



SUMMARY OF THE DERIVATIVES OF THE GERM LAYERS 



Endoderm. 

 Oesophagus 

 Crop 

 Gizzard 

 Stomach-intestine 



Ectoderm. 

 Outer epithelium 

 Nervous system 

 Stomodaeum 

 Proctodaeum 

 Ends of nephridia 



""TStesoderm. 

 All muscles 



Endothelium of coelom 

 Chlorogogue cells 

 Calciferous glands 

 Blood vessels 

 Dissepiments 



Nephridia, functional parts 

 Seta sacs 

 Reproductive organs 



