46 



POl^ARITY AND SYMMETRY; 



positely directed gradients in this case, one animal-vegetative, 

 and one vegetative-animal gradient. The fate of each cell will 

 be determined by the interaction of the two. Addition of certain 

 chemicals to the sea water surrounding the germ may produce 

 an ascendancy of one of the two gradients over the other. 

 Lithium ions suppress the animal gradient so that the vegetative 

 differentiation of the embryo begins to preponderate at the 

 expense of the animal differentiation (Fig. 16). Other sub- 

 stances, such as sodium thiocj^anate, have the opposite effect; 

 they reinforce the animal gradient at the expense of the 

 vegetative gradient so that the animal differentiation of the 

 embryo is promoted, and its vegetative differentiation inhibited. 

 Lindahl's investigations (1936) indicated that these gradients 



Fig. 16. Abnormal sea urchin embryos, produced by treatment with 



lithium, (a) exogastrula; (b-e) increasing growth of the endoderm 



at the expense of the ectoderm; ect.: ectoderm; ent.: endoderm; Tnes.: 



mesenchyme cells. After Herbst. 



