IV. i INITIAL STRUCTURE OF THE GERM 189 



A- 



FIG. 97. A and C, formation of ex-ovates in the egg of Arbacia by 

 dilution of the sea-water ; K, nucleus ; m, egg-membrane. B and Z>, 

 blastulae formed from A and C; B becomes constricted into two blas- 

 tulae, each of which gives rise to a Pluteus ; D produces a single Pluteus. 

 (After Loeb, from Korschelt and Heider.) 



