I \<x, \-I RULATION AND RELATED MODIFICATION'S. 6l 



of metabolic activity and of other associated properties and 

 conditions (COo production, O 2 consumption, electrical pott-nti.il, 

 vital staining capacity, etc.) has been demonstrated by the help 

 of many different method- (Child, '156 '20). Of these nu-thods 

 ih.M of determining the relative susceptibility of various levels 

 along i IK- -irintur.il axis is most readily applicable in embryos, 

 and ha- been found a generally reliable, if somewhat gross, in- 

 dicator of the presence and nature of the physiological gradients, 

 the -ignith -.nice of which for development has been emphasized 

 by thi- writer. 



Mt-thods. As applied to ontogenetic studies there .ire really 

 interrelated susceptibility methods: (a) direct determination 

 of -ii-< eptibility of different parts of the egg or embi\o; .m.l 

 differential inhibition, differential acclimation and diffi-reiui.il 

 i, overy in development. 



ents in concentrations determined to be lethal within a 

 few hours commonly cause death and disorganization tir-t of the 

 part- of the egg or embryo with most rapid metabolic acthiiv 

 and then later attack in succession in the same way tho-e portion* 

 \\ith progressively lower rates of metabolism the susceptibility 

 paralleling roughly the metabolic gradient (direct susceptibilit\ }. 

 In lower concentrations which kill after a day or more the on Id- 

 ol -n- rpiibility was found to be reversed, since regions with 

 IIH-IV active metabolism acclimate more rapidly and complc 1 

 (indirect susceptibility). For these purposes a \a-t nnmbd of 

 lethal a^ent- have been employed with generally concordant 

 rc-ult-, but the more toxic agents, such as cyanide*, ane-theti.-, 

 >ah- of hea\\ met.ils, basic dyes, etc., usually give be-t re-ult*; 

 and \\ith >oine of the-e tests were made on the direct -n-cepli- 

 bility of the lithium-modified as compared with normal embi 



(p. 73)- 



When developmental stage- are treated \\ith the -aim- 



agents in parti. illy lethal concentration- or -ublethal coiu-i-ntra- 

 tions pemiitting acclimation or tolerance, or are returned to the 

 normal medium after temporary e\po-nrc. a dirfeieiiti.il effect 

 on development results. That i-. the le\d- of hi-he-t metabolic 

 rate in any gradient are more retarded or inhibited, or in case of 

 acclimation or reco\ er\ . acclimate or reo\er more rapidly or 

 more completely than level-, of lower rate. In differential in- 



