THE PHYLUM ANNELIDA 



regeneration in planarians, p. 322). Tubijex can regenerate only three or four 

 somites at its anterior end, and neoblasts apparently do not migrate into the 

 anterior regenerating region from other parts of the body. The importance 

 of neoblasts in the regeneration of Tubifex has been confirmed by experiments 

 in which injured worms were exposed to X-radiation. When the worms are 

 given a suitable treatment with such radiation, the neoblasts are selectively 



. Spermatozoon 



Polar bodies 



Polar bodies 



Ectoderm 



Blastocoel 



Ectoderm 

 Mesoderm 



Blastocoel 



Mesoderm 



Posterior 



Endoderm 



Posterior 



/Mesoderm 



^Blastopore 

 (anterior) 



Nephridia 



Fig. 14.13. Development of the earthworm. A and B, penetration of the ovum by a sperma- 

 tozoon and activation of the maturation process in the ovum. C, fusion of male and female 

 pronuclei; note the extruded polar bodies. /), two-cell cleavage sta^e. E, section of an early 

 blastula, showing the blastocoel and the cells that will ejive rise to ectoderm, endoderm, and 

 mesoderm in later development. F, parasagittal section of a late blastula, showing particularly 

 the proliferation of mesodermal cells from the mesoblast into the blastocoel. G, frontal sec- 

 tion of an embryo (late gastrula); note the relationship of the developing coelomic cavities to 

 the old blastocoel, in the typical schizocoelous method of coelom formation. H, late embryo, 

 in which the basic organization of the organ systems of the future adult is recognizable; the 

 epidermis and parts of the nervous system have arisen from ectoderm, the gastrodermis has 

 arisen from endodermal cells, and structures between gastrodermis and epidermis are generally 

 of mesodermal origin. (Based on figures and descriptions by E. B. Wilson, 1889, Journal of 

 Morpholoay, vol. 3.) 



413 



