IV. i INITIAL STRUCTURE OF THE GERM 



215 



vegetative pole after fertilization. It becomes attached entirely 

 to one of the first two blastomeres, namely to CD, into which it 

 is withdrawn ; prior to the next division it is again protruded, 

 attached to D only, and then once more withdrawn : at the time 

 of the next division it is protruded again, for the last time 

 (Figs. 121, 122). 



Crampton has isolated the blastomeres of I/yana&sa and watched 

 their development (Figs. 123-6). , f, | (A, B, and C), $, f 



d 



FIG. 122. Normal cleavage of Hyanassa. (After Crampton, 1896.) 

 a, 8-celled stage from above ; 6, IG-cellecl stage from above ; c, 24-celled 

 stage, 2 from above ; d, optical section, slightly oblique, of a later 

 embryo, from below. Division of mesoblast pole-cell (d 1 ). 



(a macromere and a micromere), f, | (a micromere), -f^, -^ and 

 YG all continue to segment as though the missing blastomeres were 

 present, except for certain small irregularities in the positions of 

 the cells ; the A, B, and C blastomeres, separately or together, bud 

 off the usual four generations of micromeres, but D only the first 

 three; the division of the micromeres continues in nearly normal 

 fashion. The i micromere divides twice only. All die, however, 



