EMBRYOLOGY OF THE EARTHWORM 161 



sepiments 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 endothelium, 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 primi- 

 tive 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 digestive 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 meso- 

 derm, as are also the blood vessels, muscles, reproductive organs 

 and seta sacs. The young worm is now ready for an indepen- 

 dent life, and it leaves the cocoon after from two to three weeks. 



SUMMARY OF THE DERIVATIVES OF THE GERM LAYERS 



Endoderm. Ectoderm. Mesoderm. 



Oesophagus Outer epithelium All muscles 



Crop Nervous system Endothelium of coelom 



Gizzard Stomodaeum Chlorogogue cells 



Stomach-intestine Proctodaeum Calciferous glands 



Ends of nephridia Blood vessels 



Dissepiments 



Nephridia, functional parts 

 Seta sacs 

 Reproductive organs 



