MODIFICATION OF DEVELOPMENT IN THE FROG A481 
Differential acceleration. Those types of deviation from the 
usual trend of development that are characterized. as differential 
acceleration involve the same regions that are distorted in in- 
hibited development, but the changes are opposite in direction. 
Regions of the egg, normally most active, become, under condi- 
tions that markedly accelerate development, even more preco- 
cious. As seen in later development, the conspicuous feature of 
accelerated development is a megacephalic condition of the em- 
bryo in which the gill plates and ventral suckers are unusually 
prominent. There is also a tendency toward a dorsal convexity. 
It may be remarked in passing that differential acceleration 
forms do not deviate from the usual trend of development to 
anything like the extent seen in inhibited development—which, 
from the nature of the case, could hardly be otherwise. 
Differential acceleration differs from acclimation and recovery 
in that the embryos develop more rapidly than the controls—a 
circumstance that is especially noticeable during the early cleay- 
age stages when the susceptibility is still relatively low. Not 
infrequently eggs may be accelerated during early cleavage stages 
and severely inhibited or killed in the same solution later in de- 
velopment. In mercuric chloride, e.g., eggs may be considerably 
accelerated during the first two or three cleavages in m,/10,000 
and killed during gastrula stages in m/2,000,000. 
Acceleration is seen to a greater or lesser extent in low concen- 
trations of all of the substances used, but no special efforts were 
made to determine the exact conditions for its appearance with 
placed in the solution. Treatment: 12 hours in HCl + 81 hours in water. The 
‘fused’ suckers can be seen as cone-like structures protruding from the anterior 
ventral region. 22, lateral view of an embryo, to show the marked dorsal con- 
cavity frequently seen in differential inhibition forms. Olfactory pits, ventral 
suckers, and probably optic vesicles ‘fused.’ Eggs late cleavage when placed in 
the solution. Treatment: 3 hours in m/5 LiCl + 8 days in water. 23, lateral 
view of spina bifida embryo. Eggs two-cell stage when placed in the solution. 
Treatment: 4 days in m/10,000 KNC + 8 days in water. 24 to 26, eggs in early 
gastrula stages when placed in the solution. Treatment: 3 hours in n/300 NaOH 
+ 6 days in water. Ventral suckers and nasal pits ‘fused.’ 26, dorsal view of a 
spina bifida form with a persistent yolk plug. ¢, tail; y.p., yolk plug; v.s., ventral 
sucker. 10, 11, 23, from Exp. KNC-C. 9; 12 to 14, from Exp. IV 62;15 to21, from 
Exp. 103.3; 22, from Exp. IV 58; 24 to 26, from Exp. 125.2. 
