748 PATTERNS AND PROBLEMS OF DEVELOPMENT 



effect at different levels of a gradient also differs with agent. With KCN it is relatively 

 slight; with CUSO4 and HgClz, considerable, at least for Arbacia; with LiCl, relatively 

 great for all forms used. These differences are doubtless related to the particular 

 way in which a given agent brings about its effect. 



KCN has been used in many concentrations from m/ioo with brief temporary ex- 

 posures to m/ 200,000, m/3 00,000, and m/400,000 with continuous exposure. Ethyl 

 alcohol, 3-4 per cent, is gradually lethal for Arbacia, but with temporary exposure 

 differential inhibition is followed by rapid and extreme differential recovery; dif- 

 ferential tolerance and conditioning following inhibition appear in 2 and i per 

 cent {Arbacia). Rapid and extreme secondary modifications follow the initial inhi- 

 bition in sea water to which certain concentrations of acetic or hydrochloric acid are 

 added, the effective agent undoubtedly being CO2 set free by the acid. In earlier experi- 

 ments with Arbacia the pH was not determined, but it was determined later that the 

 concentrations of acid added were not high enough to make H-ion concentration effec- 

 tive. Sodium hydroxide from m/i,ooo to m/5,000 inhibits differentially, but with 

 secondary conditioning in the lower concentrations and recovery in water {Arbacia). 

 Primary differential inhibition and secondary modifications occur in hypotonic sea 

 water (go and 75 per cent. CUSO4 from m/ioo,ooo to m/2, 500,000 inhibits dif- 

 ferentially; rather extreme forms of differential recovery follow returns to water from 

 even the higher concentrations; and with continuous exposure to the lower concen- 

 trations forms showing marked degrees of differential tolerance or conditioning ap- 

 pear. HgClj m/i, 000,000 is strongly inhibiting differentially and finally lethal for 

 Arbacia; in m/5,000,000 and m/io,ooo,ooo there is primary differential inhibition 

 with slight secondary modification in the higher concentration in 10-20 per cent 

 of the culture, in the lower concentration in 80-90 per cent, forms with large oral lobe 

 and wide brachial angle resulting. 



Lithium chloride has been used in a large number of concentrations from m/200 to 

 m/io and with exposure periods ranging from i or 2 hours to continuous throughout 

 development and beginning at different developmental stages; experiments have 

 been repeated again and again with different lots of eggs, with the same concentrations 

 and exposures and with slight variations. Arbacia punctiilata, Strongylocentrotiis 

 purpuratus, S. franciscanus, Echinarachnius parma, Dendraster cxccntricns, Asterias 

 forbesii, and Patiria miniata have all served as material for LiCl experiments. The 

 special interest in this agent results primarily from its high effectiveness in produc- 

 ing exogastrulation; this has led many to regard it as regionally specific in action. In 

 the writer's experiments LiCl has been used not only in the analysis of exogastrula- 

 tion but for production of other modifications. With many concentrations effects of 

 approximately isotonic solutions and of sea- water solutions which are somewhat hyper- 

 tonic have been compared on the same material. In the high concentrations, which 

 sooner or later stop development or are lethal, the hypertonic solutions are somewhat 

 more effective in modifying development differentially; with temporary exposure they 

 usually give a higher percentage or more extreme type of exogastrulation than the 

 isotonic solutions; with lower concentrations no distinct difference in effect has been 

 observed. 



According to MacArthur (1924), m/i6o is about the optimum for exogastrulation in 

 Echinarachnius. Higher concentrations are required for Dendraster; 100 per cent or 



