DIFFERENTL\L DEVELOPMENTAL MODIFICATION. Ill 255 



primarily on a differential susceptibility of different embryonic region, 

 which is, to a high degree, similar for different agents though the differen- 

 tial in effect may be greater with some than with others. In many cases, 

 however, analytic interpretation of the experimental data is difficult or 

 impossible because of experimental procedure. The range of concentration 

 or intensity used is often narrow; exposure to the agent is sometimes con- 

 tinuous, sometimes temporary, and, in certain experiments, repeated at 

 intervals, and effects of exposures beginning at different developmental 

 stages are not always clearly distinguished. The possibility of differential 

 tolerance or conditioning to low concentrations or intensities and of dif- 

 ferential recovery after temporary exposure has received no attention from 

 most authors. In most of the experiments with earlier stages susceptibility 

 decreases from the prospective anterior region posteriorly, but sooner or 

 later a second region of high susceptibility appears at a more posterior 

 level. In the fishes this occurs earlier in some species used in experiment, 

 later in others (see Fig. 52, p. 149). In the amphibians the dorsal lip region 

 has become highly susceptible at the beginning of gastrulation (Fig. 53, 

 p. 152), and in the chick the region of the primitive streak from early 

 stages to its disappearance is highly susceptible (Hyman, 1927a). Differ- 

 ential inhibition of the earlier stages always involves the head region to a 

 greater or less degree ; but modifications resulting from inhibition of the 

 region of gastrulation may be absent, differ widely in degree, or appear at 

 different body-levels, according to stage at which exposure begins, period 

 of exposure, and rapidity and degree of action of agent, and possible indi- 

 vidual differences in susceptibility. A variety of modifications may result 

 from differential inhibition in a single experimental lot; and, if secondary 

 modifications of conditioning or recovery are possible, the range of forms 

 becomes still greater. Some have maintained that there is no discernible 

 law and order in the teratogenic action of external agents (e.g., Kellicott, 

 1916; Cannon, 1923). As a matter of fact, however, law and order are 

 becoming increasingly evident as experiment and analysis continue; but 

 it is only by working with a considerable range of concentrations or in- 

 tensities, exposure periods, and developmental stages that we can hope 

 to attain definite results. 



Numerous agents have been used in experimental vertebrate teratog- 

 eny : for example, magnesium salts, lithium salts, cyanides, inorganic and 

 organic acids, bases, various other electrolytes, anesthetics, alkaloids, 

 acetone, formaldehyde, low and high temperature, ultra-violet radiation, 

 X-rays, and radium. In most experiments the agent used has been ap- 



