ary 
; 
- 
ARTEMIA SALINA 321 
placed in fresh culture medium with fresh food. In the first experiments 
the surviving nauplii were counted and transferred daily into fresh medium. 
Comparisons with controls showed that the effect of arsenic treatment 
ceases on the third day after treatment. After three days the mortality of 
treated nauplii is quite small and does not exceed that of the controls. 
Therefore the percentage of survivals after this period has been taken as 
a measure of resistance. 
The response of different broods of the same parentage was found to be 
significantly heterogeneous in some cases. Whether this irregularity is 
due to unequal resistance in different broods, or to differences in the con- 
ditions on which they were tested, has not yet been established. Changes 
in the stock solution of sodium arsenite are not likely to occur. The possi- 
bility that differences in the composition of the soil extract might be the 
reason for the irregularities and that the method could be improved by 
using sterile sea-water without soil extract for the 24-hour tests was explored. 
Although Artemia does not breed well in sea-water, the nauplii do not 
suffer to any visible degree if kept therein for 24 hours. (Traces of soil 
extract are introduced by 5 cc. of the Chlamydomonas culture which can be 
only grown in ‘ Erdschreiber.’) ‘These experiments show in two cases out 
of seventeen considerable differences in the number of survivals in ‘ Erd- 
schreiber ’ and in sea-water which most probably have been caused by the 
presence and absence of soil extract respectively. But some tests show that, 
using sea-water only, the whole source of heterogeneity has not been removed. 
The data indicate, however, that when sufficient numbers are used, 
reliable resistance curves can be obtained. It is further clear that parallel 
families in the bisexual races show consistently different powers of resistance. 
With respect to any increase in resistance due to treatment and selection 
the data so far are necessarily very inadequate. Two comparisons seem 
worth making. In the parthenogenetic line E4, we may compare the aggre- 
gate responses of 322 nauplii from untreated parents with those of 408 from 
treated parents. 
PERCENTAGE SURVIVAL. 
Number Number 
Strength. Untreated. Tested. Treated. Tested. 
% % 
0°38 100 31 — — 
0°40 100 3I -— —_ 
o'41 79 48 — = 
0°42 67 12 — — 
0°43 53 81 100 29 
0°46 43 53 100 29 
0°50 20 66 38 IIo 
0°52 = = 23 86 
0°53 = i= 2 45 
0°55 _ — 20 64 
0°56 as = 9 45 
322 408 
A second comparison may be made for the bisexual lines Cr, C2, C7 and 
Cog, all of which have some broods after one and some after two genera- 
_ tions treatment. Of these 1,120 have had one generation treatment and 
824 two generations. 
