SPONGES, FLAT WORMS, AND ROUND WORMS 157 



FIG. 79. Development of Planaria lactea. i, egg (0) surrounded by yolk (v) ; 

 2, four blastomeres from segmented egg ; j, later stage ; more blasto- 

 meres (bl) ; 4, much later stage, differentiation of blastomeres into ecto- 

 derm (ep\ entoderm (hy), a provisional pharynx (pti), and wandering 

 cells (w) ; 5, cellular differentiation more advanced; ep, ectoderm ; 

 ent, primitive gut ; hy, entoderm ; ph, pharynx ; 6, embryo changes 

 shape to a flattened ovoid, ent, primitive gut: m, mouth; ^phar- 

 ynx. (From Lankester after Hallez.) 



Regeneration. Planarians show remarkable powers of re- 

 generation. If an individual is cut in two (Fig. 80, A), the an- 

 terior end will regenerate a new tail (B, B'), while the posterior 

 part develops a new head (C, CO- A crosspiece (D) will regener- 

 ate both a head at the anterior end, and a new tail at the posterior 

 end (D'-D 4 ). The head alone of a Planarian will grow into an 

 entire animal (E E 3 ). Pieces cut from various parts of the 

 body will also regenerate completely. No difficulty is experi- 

 enced in grafting pieces from one animal upon another, and many 

 curious monsters have been produced in this way. 



