82 Evolution and Adaptation 
blastula, and gastrula stages,” etc. ‘“ Moreover, the embryo 
may adapt itself to special conditions of embryonic life, and 
produce organs of an ephemeral nature like the amnion, 
chorion, and placenta.” 
“A comparison of ontogenetic with antecedent phylo- 
genetic stages must always keep in view the fact that the 
action of external and internal factors has brought about 
considerable changes in the ontogenetic system, and, indeed, 
in a generally advancing direction, so that in reality a later 
condition can never correspond to a preceding one.” 
Hertwig sums up his conclusion in the statement that 
ontogenetic stages give us, therefore, a greatly changed 
picture of the phylogenetic series of adult ancestors. ‘ The 
two correspond not according to their actual contents but 
only as to their form.” Hertwig also repeats His’s idea, that 
the reason that certain kinds of form repeat themselves in 
the development of animals with a great constancy depends 
principally on this, that they supply the necessary conditions 
under which alone the following higher stage of the ontogeny 
can be formed. The development, for instance, begins with 
the division of the egg, because this is the only way that a 
one-celled condition can give rise to a many-celled form. 
Again, the organs can be formed only when groups of cells 
have made a closer union with one another. Thus the gastrula 
must begin with the antecedent blastula, etc. Definite forms 
are, despite all modifying influences, held to firmly, because 
by their presence the complicated end-stages can be reached 
in the simplest and most suitable way. 
Thus Hertwig adopts here a little from one doctrine and 
there a little from another, and between his attempt to reinstate 
the old biogenetic law of Haeckel, and to adopt a more modern 
point of view, he brings together a rather curious collection of 
statements which are not any too well coordinated. Take, 
for example, his description of the relation between Haeckel’s 
gastreea and the embryonic gastrula stage. The latter he 
