354 



ESSENTIALS OF ZOOLOGY 



and anus. The mesodermal cells which fill the space between the 

 ectoderm and endoderm develop segmental cavities which are the 

 coeloms of the metameres. This constitutes a tube within the worm 

 from which eventually develop the organs of the adult individual. 



Blastocoe/ 



flesob/ast 

 cell 



tl^soblast 

 tissae 



hesoblast 

 cell 



Ectoderm A 



fndoderfT)' 



flesob/ast 

 cell 



Unas . 



Ectoderm 



Endoderfi) 



Ectoderm 

 Coelom 



Hesoblast 

 Cijme 



3of7)atic 

 esoderm 



EododeriT). 



Coelom 



Splaachnk 

 mesodau) 



Ectoderm 

 Have cord 



Endoderm 



Coelom'ic 

 cavities 



hesodermk 

 tissue 



Ectoderm 



i^noatl? 



Fig. 131. — 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; C and D, showing- stages in the beginning 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, gastrula in 

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

 mouth and arms ; J, same as I but in cross section ; K, cross section of later stage. 

 (After Wilson, Embryology of the Earthworm, Journal of Morphology, 1889.) 



