DIFFERENTIAL DEVELOPMENTAL MODIFICATION. II 215 



a crowded lot; loss of epithelial character and dissociation of entoderm 

 are evidently not specific effects of a particular agent : they depend on con- 

 centration and exposure period. The apparent asymmetry of C and D 

 results from ventrodorsal differential inhibition. The inhibiting conditions 

 may completely prevent development of ventrodorsality (Fig. 85, E, F); 

 but if it develops, inhibition decreases from the ventral region dorsally. 

 That it is actually the ventral region that is more inhibited cannot be de- 

 termined from forms like C and D, but many individuals undergo suffi- 

 cient development to permit certain identification of ventral and dorsal 

 regions.** Gastrulae such as B and D of Figure 85 usually show little or 

 no further development; but forms like A, C\ E, and F may continue 



Fig. 86, A-D. — Slight secondary modifications in Patiria after relatively extreme inhibi- 

 tion. A, KCN m/50,000, 18 hr. from two-cell stage, 11 days water; B, LiCl m/20, 16 hr. 

 from two-cell stage, 8 days water; C, LiCl m/30, 38J hr. from early cleavage, 8 days water; 

 D, LiCl, m/25, 19J hr. from early cleavage, 5 days water. 



development as differentially inhibited forms or show some secondary 

 modification, according to experimental conditions. 



The question arises whether the cells dissociating from the entodermal 

 region represent premature mesenchyme, and, if so, how it is possible 

 that inhibiting factors determine their appearance. The data on dye re- 

 duction indicate low oxygen in the blastocoel ; in the starfish it is so low 

 that the apical region of the archenteron, from which mesenchyme nor- 

 mally dissociates, reduces both methylene blue and Janus green in water 

 of the usual oxygen content and open to the air (Child, 1936a). It is sug- 

 gested that under inhibiting conditions which decrease oxygen or inhibit 

 its intracellular utilization, dissociation may occur without activation of 

 the region concerned or with slight activation. But whether dissociated 



^ Forms like those of Fig. 85 also appear in low oxygen, in hypotonic sea water, with Janus 

 green, and with ultra-violet radiation. 



