gO PATTEN. [Vor. XET. 
energy throughout the three embryos, so that each one gets an 
equal share, there will be a tendency to bring all three embryos 
down to the same grade of degeneration. We may thus explain 
the diminished difference between the three embryos in Fig. 103 
and those in 102; and finally in Fig. 104, where the three embryos 
have evidently been formed a long time, all three are reduced 
to nearly the same condition. At first sight these conclusions 
seem to be in direct opposition to the fact that in single 
asymmetrical embryos ¢here 7s no indication whatever that the 
stronger half possesses any power by virtue of which missing 
parts on the opposite side of the median line can be restored. 
If one embryo can produce two new halves by drawing on 
its bank account, as in the formation of double and triple 
embryos, why do not defective single embryos restore an 
absent half or quarter? We can only say that in such 
cases the right half of the embryo, for example, was absent 
because the formative material for that part was absent 
from the start. Theoretically there is no reason why the 
lost part should not be restored by a forced draft on the 
corresponding organs of the whole side. Aut there is no 
particular reason for assuming that forced growth would be 
likely to occur in an embryo that was already defective in 
growth material. 
On the other hand, it is a point to be borne in mind that in 
all the defective embryos shown in Pl. IV very few show a 
defect at the anterior end of the body, unless, as in Fig. 38, 
the whole left half is absent, or in Fig. 37, which is probably 
a double embryo. The parts most frequently absent are the 
posterior ones, or ones across the middle of the thorax. That 
as, they are the parts least likely to be restored from the opposite 
side, provided their restoration took place in the same order that 
the organs of new halves are formed in double and triple 
embryos. 
According to Hertwig and others, the stimulus of a modified 
environment alone is sufficient to call forth the new embryo. But 
as in the cases we are considering the environment was nearly the 
same for all, it can only mean that out of many thousands of eggs 
the few that produced double or triple monsters were different at 
