METABOLIC GRADIENTS 59 



posterior region of the body while in the anterior region 

 development proceeds more or less normally. In such 

 cases the posterior regions, which possess a lower meta- 

 bolic rate than anterior regions, do not acclimate to the 

 conditions as readily as the latter and are therefore 

 retarded or inhibited to a greater extent in their develop- 

 ment. Such embryos produce certain characteristic 

 forms of monsters, more or less completely normal 

 anteriorly and increasingly abnormal in the posterior 

 direction. Where acclimation does not play a part the 

 anterior regions of the embryo may be most, the posterior 

 least, affected and another type of monsters results. In 

 many of these monstrous forms the symmetry gradients 

 as well as the major gradient appear more or less clearly. 



In fact the field of teratogeny, the experimental pro- 

 duction of monstrous or abnormal forms, contains a 

 large amount of evidence for the existence of suscepti- 

 bihty gradients, but neither the relation between sus- 

 ceptibility and metabolic rate nor the existence of the 

 metabolic gradients has been recognized by the investi- 

 gators in this field. There is no doubt that further 

 experiments directly concerned with the problem of 

 susceptibility and metaboHc gradients will afford even 

 more definite and positive results. 



These gradients in susceptibihty indicate the ex- 

 istence in the animal organism of more or less definite 

 metabolic gradients essentially quantitative in nature. 

 In other words, we find a definite order in the gradation 

 of rate or intensity of general metabolic activity in 

 directions coinciding with those in which an orderly 

 sequence of events and arrangement of parts or an 

 orderly behavior of the organism in other respects are 



