336 FIELDS AND GRADIENTS IN NORMAL ONTOGENY 



produced, which are entirely composed of endoderm, with the 

 usual exception of a tiny button at the apical end of the large endo- 

 dermal vesicle. When the treatment is discontinued at the 24-hour 

 stage, only moderate effects, resulting in exogastrulae, are found. 



En. 



Fig. 156 

 Differential inhibition of the ectoderm in sea-urchin larvae reared in water to 

 which lithium salts are added. Progressive stages of inhibition with increasing 

 concentrations of lithium, a, is an exogastrula; e, is completely endodermised. 

 Ek. ectoderm; En. endoderm; Mz. mesenchyme; Pz. pigment cells; Zr. clump 

 of cells at base of exogastrulated gut. (After Herbst, from Schleip, Determina- 

 tion der Pri?mtiventzvicklimg , 1929, fig. 323, p. 505.) 



When the treatment is not begun until the late blastula or gastrula 

 stage, death soon ensues, but without any modification of the pro- 

 portions of the germ-layers, indicating that this is determined by 

 the mid-blastula stage. 



It may be suggested that this result is in part due to differential 



