62 JOHN W. MACARTHUR. 



hibition then the levels of lower rate may be relatively accelerated 

 and in differential acclimation or recovery the levels of higher 

 rate may be relatively accelerated after the primary inhibition. 

 Direct differential acceleration is also possible under proper con- 

 ditions. These differential effects of external agents in relation 

 to the physiological gradients provide means of controlling to a 

 greater or less extent the course of development and the sizes, 

 proportions and relations of parts under given conditions (Child, 

 '16, '17; Bellamy, '19). 



In short, the established gradient of the egg or embryo, being 

 physiological, is not necessarily permanently fixed and unalter- 

 able, but may be radically modified, obliterated entirely (as in 

 Asterias embryos by N/iooo HC1 in sea water), or even entirely- 

 reversed in direction, as we shall show for the lithium larvae 



(Fig. 13). 



The ordinary procedure in the differential inhibition method 

 was as follows: Eggs of freshly collected animals were removed 

 to a bowl, fertilized, and then placed during early cleavages in 

 small quantities in liter Erlenmeyer flasks of sea water to which 

 the agent used had been added. The flasks were loosely covered 

 but not aerated. Each day the solution was renewed and em- 

 bryos examined and notes and sketches taken recording the 

 course of their development as compared with the controls kept 

 in pure sea water. In most experiments some of the embryos 

 were also removed after various times of exposure to the salt 

 solutions to determine the possibility and nature of the steps of 

 their recovery. 



The Lithium Modifications. Best results with lithium are 

 contingent upon recognition of the prominent roles of several 

 factors; especially concentration of the agent, duration of ex- 

 posure, and stage of development at time of exposure. In higher 

 concentrations development is strongly inhibited and incomplete. 

 If the concentration is too low, or if exposure is too brief, begins 

 too late or is terminated at too early a stage, tin- effect is weak 

 and development approaches by all transitions toward:- tin- 

 normal. Herbst observed ('950, ch. 3) that typical eflect> are 

 not produced by temporary treatment merely up to fertilization, 

 cleavage or young blastula stages; but well developed blastulae 

 treated with rather concentrated solutions and then returned to 



