MODIFICATION OF DEVELOPMENT IN THE FROG A477 
tions be inhibiting, accelerating, or such that the embryo accli- 
mates to them, or, on removal to normal conditions, recovers. 
Hence one finds the distortion in development, regardless of the 
type, whether inhibition, acceleration, acclimation or recovery, 
appearing in relation to the principal axes or planes of symmetry 
of early developmental stages—anteroposterior, dorsoventral, and 
mediolateral—and to the secondary axes of special organs arising 
later in development. In early development before regional 
differentiation has begun, anterior, dorsal, and medial regions 
which are physiologically more active than their opposites, are 
more affected by disturbing conditions of the four sorts men- 
tioned above than the physiologically less active posterior, ven- _ 
tral, and lateral regions. This is the typical condition. With 
the origin of special structures, however, optic vesicles, posterior 
growing region, limb buds, e.g., or in other words, with the origin 
of the secondary physiological gradients of special organs, the 
typical condition may be considerably modified, at least tempora- 
rily. Such special structures, which, it may be mentioned in 
passing, arise in an orderly relation to the primary axes, May 
‘flare up’ with a high initial rate of metabolism that may persist 
through the period of most rapid proliferation and differentiation. 
The optic vesicles, e.g., for a time represent the most susceptible 
region of the anterior end of the embryo. | Similarly, the tail 
bud, which terminates the posterior growing region, is at the time 
of its origin quite as or may be even more susceptible to disturbing 
conditions than more apical regions. These constantly changing 
relations in the developing embryo between the primary physio- 
logical gradients and the gradients of special organs arising later 
in development, together with the individual variation in the 
physiological condition of the embryo as a whole, make absolute 
uniformity among the modified embryos, whether inhibitions, 
accelerations, acclimations, or recoveries, highly improbable, to 
say the least. Great variation is to be expected in every case. 
The four types of modifications obtained are characterized as: 
differential inhibitions, differential accelerations, differential ac- 
climations, and differential recoveries. They are described in the 
order mentioned. It is hardly practicable or even desirable here 
