242 PATTERNS AND PROBLEMS OF DEVELOPMENT 



tionably an essential factor in establishing them. Differential modifica- 

 tions of development indicate a single primary activity gradient in the 

 apicobasal axis and a secondary gradient arising later in the basal region 

 and partly obliterating the primary gradient. Analysis of developmental 

 stages of exogastrulae gives further evidence in support of this conclusion. 

 Prospective entoderm is less susceptible than ectoderm in early stages, 

 and more susceptible in later stages, to lithium and other agents. Lithium 

 and various other agents entodermize prospective ectoderm in early 

 stages, apparently by depressing or inhibiting it, but, after a certain de- 

 velopmental stage, inhibit entoderm more than ectoderm; that is, their 

 action is inhibitory throughout. These experiments do not prove the ab- 

 sence of the gradients postulated by Runnstrom but indicate a different 

 sort of gradient pattern as the primary effective factor in determining 

 form and proportions, and alterations of form and proportion are often 

 associated with changes in localization and differentiation of parts, indi- 

 cating that these factors of development originate in the same pattern. 

 Evidences of specific regional and local differences certainly become in- 

 creasingly evident as development progresses, but it does not necessarily 

 follow that they are the primary features of pattern. 



Runnstrom has used potassium-free sea water, ZnS04, lithium, CO, and 

 other agents in modifying echinoid development; but determination of the 

 effects of a wide range of concentrations of the chemical agents and of 

 different exposure periods at different stages of development does not 

 appear to have been undertaken to any great extent. Apparently, neither 

 the possibility of change in susceptibility of prospective entoderm in the 

 course of development nor that of secondary developmental modifications, 

 opposite in character to the primary, in low concentrations of external 

 agents or in recovery after return to water, has been recognized. Conse- 

 quently, it is often difficult to determine whether forms described are direct 

 effects of the agents used or secondary modifications. The experiments 

 with potassium-free sea water suggest several possibilities: distribution of 

 potassium may differ regionally in the egg or embryo; with lack of ex- 

 ternal potassium internal distribution may be altered; and the effect on 

 development of absence of an essential ion, such as potassium, may be 

 different from that of a chemical or physical agent acting in addition to 

 the natural environment. Experiment with different amounts of potas- 

 sium, both below and above the content of natural sea water, appear 

 desirable as an adequate basis for analytic interpretation. 



In several papers Lindahl and others have presented voluminous experi- 



