GRADIENT-FIELDS 



45 



with a large gut, and an irregular skeleton, but without either 

 apical tuft or band of cilia (Fig. 15). Evidently, the potency 

 for the formation of a gut decreases from the vegetative to- 

 wards the animal pole, whereas that for the formation of an 



Fig. 15. Sea urchin larvae, (a) normal gastrula and (b) normal 

 pluteus; (c) embryo with a large tuft of cOia, produced by a frag- 

 ment of the animal side of an egg; id) embryo containing gut and 

 skeleton formed by a fragment of the vegetative side. After 



Horstadius. 



apical tuft decreases in the opposite direction. When cell groups 

 from different parts of the germ are brought together, more 

 or less normal larvae will result even from highly abnormal 

 combinations of blastomeres, so long as cells with animal and 

 cells with vegetative potencies are present in balanced 

 proportions. 



Runnstrom (1928) has suggested that there are two op- 



