86 PATTEN. Vou EXD; 
(2) There is a great difference in the growth period under 
apparently the same conditions. 
(3) There is a great difference in the size of different 
embryos, some being much larger than the normal and others 
smaller. 
(4) Certain organs or regions of the body may be entirely 
absent and are not subsequently restored. 
(5) When organs once formed disappear in certain regions, 
it is usually by median fusion and degeneration in the reverse 
order of their age and specialization. 
(6) Multiple embryos are due to the formation of new parts, 
which appear in the reverse order of that in which old organs 
disappear by median fusion and antero-posterior degeneration. 
(7) Multiple embryos thus formed quickly disappear again 
by median fusion and antero-posterior degeneration. 
(8) Individuals of triple embryos recently formed differ 
greatly in size and in the amount of degeneration. 
(9) In old triple embryos the individuals are more nearly alike. 
Taking up each set of facts, except the first, it would seem 
from a consideration of the variations of the second class that 
every individual and every part of it has a definite vate and range 
of growth, a rise and a decline like a time clock that has been 
set to go a definite period at a definite rate. The time for the 
whole embryo may be reduced apparently to almost any frac- 
tion of the normal one, as in those that die a natural death 
before reaching stage C’; or individual organs may die and dis- 
appear by median fusion and degeneration before the other 
organs have completed their development ; and defective parts 
either remain defective, or dwindle and disappear in the midst of 
plenty, side by side with healthy flourishing organs. Embryos 
six or eight months old are found in the same stage, to all 
appearances, as normal ones only six or eight weeks old. 
Both these kinds of variation are apparently best explained 
by assuming a primary variation in the quantity or quality 
of growth material, or of both. Either variation may ex- 
press itself as a variation in the intensity of the “growth 
force” that may be measured in terms of the range of develop- 
ment, that is, the number of intermediate stages produced, or 
