218 



INTERNAL FACTORS 



IV. i 



In the latter genus the development of the isolated blastomeres, 

 especially of those resulting from later divisions, has been minutely 

 followed. Each separate first qiiartette cell (1 a 1 d) forms 

 (Fig. 128) a closed ectoblastic structure with an apical organ at 



FIG. 125.Ilyctnacsa : cleavage of blastomere (A), a, f, from side ; 

 b, j\, from side; c, passage to T %, first form, from side; d, resting -*$, 

 second form, from side ; e, passage to - 2 \, from side ; /, ^, from side. 

 (After Crampton.) 



FIG. 126. Ilyanassa: a c, cleavage of J blastomeres; a, s stage, 

 from one rnicromere ; b, j 4 s stage, from two micromeres; c, ^> g stage, 

 from three micromeres. d-f, cleavage of blastomeres ; d, if embryo, 

 from below: e, origin of fourth quartette, from below; /, embryo of 

 48 hours. (After Crampton ) 



