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 Hacckel, 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 

 gastraea and the embryonic gastrula stage. The latter he 



