MODIFICATION OF DEVELOPMENT IN THE FROG 495 
(b) 4 days in KNC, plus 4 days in water. Seventy per cent 
hatched, others dead in membranes without elongating. The hatched 
animals seem to have rather large heads. 
(c) 4 daysin KNC, plus7 daysin water. Actively swimming 
embryos assuming the regular tadpole form. Smaller than control, 
but with relatively larger heads—showing a differential recovery. 
(d) 4 days in KNC, plus 8 days in water. Nearly all nor- 
mal tadpoles except that the heads are relatively larger. A few spina 
bifida and microcephalic forms remain alive. Figures 69, 70 illustrate 
two of the very few—about 1 per cent—extremely modified embryos. 
They are microcephalic and bent dorsally. In one, shown in 80, the 
ventral suckers are completely ‘fused,’ but only partly fused in the 
other, 81. 
If the eggs are placed in m/10,000 KNC in late gastrula stages, de- 
velopment proceeds slowly for several days and ceases with the embryos 
dying in early neural fold stages. 
In m/50,000 and m/100,000 KNC there is usually some acceleration 
of development and many of the embryos appear somewhat megace- 
phalic. The acceleration is not as great nor is the differential nature 
of the acceleration as marked as in certain concentrations of HCl (p.497). 
Figure 29 shows a camera-lucida outline of one of the more extreme 
megacephalic types. 
Experiments with formaldehyde 
Experiment IV 62. Eggs in four to eight-cell stages were treated 
as follows in 0.0075 per cent formaldehyde. 
12 hours. Early cleavage stages with pigmented and unpigmented 
cells approximating the same size. 
24 hours. Early inhibited gastrulae. 
48 hours. A few early neural fold stages and these with very shallow 
neural grooves and with persistent yolk plugs. Figures 12, 13 illustrate 
two of these. 
72 hours. 95 per cent spina bifida. Ventral suckers approximated 
(fig. 14). There appears to be some acclimation especially in the head 
region. 
133 hours. Not much change. Many dying. 
A number of concentrations of formaldehyde were tried ranging from 
0.01 to 0.001 per cent, but the concentration used in experiment IV 62 
was the only one that gave any considerable number of markedly modi- 
fied embryos in the later stages of development. ‘The percentage strength 
was calculated on the basis of 40 per cent commercial formalin. For 
example, the 1 per cent (approximate) stock solution from which the 
other strengths were made up was made by adding 1 ce. of the commer- 
cial product to 39 cc. of water. 
