22 C. M. CHILD 



at about this time and a large third body-segment differentiates. 

 Since this growing region is the most active region of the larva 

 in the later stages acclimation is to be expected there if any- 

 where (Child '16 c). 



Since different eggs, even those from the same female, differ 

 more or less in susceptibility, there is a considerable range of 

 variation in the effect of any concentration of any agent. In 

 KNC m/100000 from the first cleavage the range is from almost 

 normal, somewhat retarded larvae, through the forms of figures 

 29 to 36 which represent the most advanced stages of the in- 

 dividuals figured, to cases where development is completely 

 inhibited at an earlier stage and death occurs in a few days. In 

 general the greater the degree of inhibition the earlier death 

 occurs. In KNC w/200000 from the first cleavage the inhibiting 

 effect is slight and the larvae are nearly all normal. In KNC 

 m/50000 development rarely proceeds beyond the stage of figure 



23 and death occurs in two or three days. 



Figures 38 to 42 show the effect of KNC where exposure began 

 twelve hours after fertilization. In figure 38, a two-day stage 

 in KNC m/50000 from twelve hours, the pretrochal region is of 

 normal size and the larva is in general not very different from a 

 one-day normal larva, the posterior growing region being evi- 

 dently completely inhibited. In this concentration the larvae 

 usually die without much further advance in development. 



Figures 39 to 41 show larval forms after four days in KNC 

 m/100000 from twelve hours after fertilization. The most 

 conspicuous features are the relatively large head and relatively 

 small body-region, a modification in the opposite direction from 

 that of figure's 29 to 33. In figure 39 the first two body segments 

 are distinguishable and bear a few setae, while in figure 40 the 

 differentiation of segments is almost, and in figure 41 entirely 

 inhibited, and no setae appear. These forms are characteristic 

 for this concentration and time. After six days in KNC most 

 of the individuals which are still alive show no further advance, 

 but some, like figure 42, show what seems to be some degree 

 of acclimation of the posterior growing region. Here the first 

 body segment is small, but the second and third segments hre 



