The Theory of Evolution 81 
so in a certain degree are recapitulated. We also admit that 
the embryonic forms of higher animals have many points of 
comparison with the mature forms of related groups standing 
lower in the system. 
“Nevertheless, a deeper insight into the conditions re- 
lating to these resemblances shows that there are very im- 
portant differences that should not be overlooked. Three 
points need to be mentioned: 1. The cell-material which in 
the ancestral chain gives the basis for each ontogenetic process 
is each time a different material as far as concerns its finer 
organization and primordia. Indeed, the differences become 
greater the farther apart the links of the original chain 
become. This thought may be formulated in another way: 
The same ontogenetic stages that repeat themselves periodi- 
cally in the course of the phylogeny always contain at bottom 
a somewhat different cell-material. From this the second rule 
follows as a consequence. 2. Between the mature end-form 
of an ancestor and the corresponding embryonic form of a 
widely remote descendant (let us say between the phylo- 
genetic gastrzea and the embryonic gastrula stage of a living 
mammal, according to the terminology of Haeckel) there 
exists an important difference, namely, that the latter is sup- 
plied with numerous primordia which are absent in the other, 
and which force it to proceed to the realization of its develop- 
mental process. The gastrula, therefore, as the bearer of 
important latent forces, is an entirely different thing from the 
gastreea, which hasalready reached the goal of its development. 
3. In the third place, at each stage of the ontogeny outer and 
inner factors are at work, in fact even more intensely than 
in the fully formed organism. Each smallest change that acts 
anew in this way at the beginning of the ontogeny can start 
an impulse leading to more extensive changes in later stages. 
Thus the presence of yolk and its method of distribution in 
the egg alone suffice to bring about important changes in 
the cleavage, and in the formation of the germ-layers, the 
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