226 



INTERNAL FACTORS 



IV. i 



from the isolated CD blastomere, then the larva has an apical 

 organ, but no post-trochal region. Hence between the first and 

 second divisions the stuff which determines the formation of this 

 organ must migrate out of the polar lobe into the animal hemi- 

 sphere of the D cell. 



In the unsegmented egg the specific material for both apical 

 organ and post-trochal region is situated in the vegetative 



FIG. 136. Dentalium. Larvae from isolated blastomeres. a, b, Twin 

 larvae from the isolated CD (a) and AB (b) halves of the same egg, 

 24 hours ; c, d, twin larvae from the isolated D (c) and C (d) quadrants 

 of the same egg, 24 hours ; e, f, twin larvae from the isolated posterior 

 micromere 1 d (e) and the 1 c micromere (/) of the same egg, 24 hours ; 

 g, \ larva from one of the A, B, or C quadrants, 72 hours. (After Wilson.) 



hemisphere. The animal half of the egg, whether removed 

 after fertilization, or before and then fertilized, segments like an 

 egg from which the polar lobe has been removed (Fig. 137 V), and 

 develops neither apical organ nor post-trochal region (rarely an 

 apical organ if the fragment is large). The vegetative hemi- 

 sphere, when cut off before the entry of the sperm and afterwards 



