80 Evolution and Adaptation 
because in its organization it contains the basis of a mammal, 
just so much more must it be different from the hypothetical 
one-celled amoeba, which has no other characteristics than 
those that go to make up an ameeba. Expressed more gen- 
erally, the developmental process in the many-celled organ- 
isms begins, not where it began in primitive times, but as the 
representation of the highest point which the organization 
has at present reached. The development commences with 
the egg, because it is the elemental and fundamental form in 
which organic life is represented in connection with the 
reproductive process, and also because it contains in itself the 
properties of the species in its primordia. 
“ The egg-cell of the present time, and its one-celled prede- 
cessor in the phylogenetic history, the amoeba, are only 
comparable in so far as they fall under the common definition 
of the cell, but beyond this they are extraordinarily different 
from each other.” 
“The phyletic series must be divided into two different kinds 
of processes : — First. The evolution of the species-cell, which 
is a steady advance from a simple to a complex organization. 
Second. The periodically repeated development of the many- 
celled individual out of. the single cell, representative of the 
species (or the individual ontogeny), which in general follows 
the same rules as the preceding ontogeny, but is each time 
somewhat modified according to the amount to which the 
species-cell has itself been changed in the phylogeny. 
Similar restricting and explanatory additions to the biogenetic 
law, like those stated here for the one-celled stage, must be 
made in other directions. Undoubtedly there exists in a 
certain sense a parallel between the phylogenetic, and the 
ontogenetic, development. 
“On the basis of the general developmental hypothesis on 
which we stand, all forms which in the chain of ancestors 
were end-products of the individual development are now 
passed through by their descendants as embryonic stages, and 
