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TEXTBOOK OF ZOOLOGY 



the anterior end to the clitellum. With their bodies in close contact a 

 closed passage is formed between the genital openings of the two in- 

 dividuals. Sperms pass from the testes out through the seminal 

 vesicles and vasa deferentia to the closed passage and move through it 



blastocoef 



t^eso blast 

 cell 



Endoderm 



Coelomk 

 cavities 



Hesodermic 

 tissue 



Ectoderm 



Fig. 110. — Development of the earthworm. A, blastula, surrounded by a mem- 

 brane ; B, section of a blastula showing blastocoele and one of the mesoblast cells 

 (primary cell) of mesoderm layer; G and D, showing stages in the beginnmg of 

 gastrulation ; E, side view of gastrula showing invagination ; F, section of gastrula 

 along a line to show polar cells, mesoderm layers on each side of them and the 

 archenteron ; G, later stage showing cavities in the mesoderm ; H, gastrxila in 

 cross section ; /, longitudinal section of a young worm after formation of the mouth 

 and anus ; J, same as I but in cross section ; K, cross section of later stage. (After 

 W^ilson, Embryology of the Earthworm, Journal of Morphology, 1889.) 



