964 THE AMERICAN NATURALIST. [Vor. XXXV. 
We have seen that there is both for pieces of the adult 
animal and for pieces of the egg, or of the isolated blastomeres, 
a lower limit of size below which a piece does not produce the 
typical form. Even in a form like Tubularia, in which a very 
small piece may produce an incomplete structure, there is still 
a lower limit below which the pieces do not produce any struc- 
ture. We find this lack of power to develop in small pieces in 
which we know from other experiments that the material is 
totipotent, and the only clue we have to account for the facts 
is that the result is in some way directly connected with the 
amount of material present. We find for each animal a lower 
limit of size —a limit that may vary at different stages of 
development —for producing a miniature copy of the type form ; 
or, in other words, the organization must have a certain amount 
of material in order to develop. We find this same law to hold 
for pieces of eggs, of embryos, and of adult Metazoa, and it is 
equally true for adult unicellular Protozoa. It has been found 
that very small pieces of the egg may continue to divide, 
although they may be much too small to produce an embryo; 
but as several other results have also indicated, the factors that 
determine the cleavage are not necessarily connected with 
those that relate to the organization of the embryo. Isolated 
blastomeres that are below a certain size also fail to produce 
an embryo. The one-eighth blastomeres and even the one- 
sixteenth blastomeres produce in some few cases the early 
stages of development, but many of them fail to do so, and 
beyond this point it is doubtful if a blastomere can develop 
past the gastrula stage. The results show that the lower limit 
is soon reached in these eggs. It is certain that for adult ani- 
mals pieces much smaller than one-eighth or one-sixteenth of 
the body can produce new animals of half size, but we have at 
present no data for comparison between the absolute minimal 
size of a piece of the egg and of the adult of the same species 
that can produce a whole animal. It has been found that 
pieces of planarians that contain less than ijs of the volume 
of the adult may produce the typical form. Pieces of hydra 
zoo of the adult produce a new hydra; and pieces of the stem 
of Tubularia about 41, of the length of the stem can also make 
