“Nevertheless, a deeper insight into the conditions relating to these resemblances shows that there are very important 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 periodically 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 phylogenetic gastræa 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 supplied with numerous primordia which are absent in the other, and which force it to proceed to the realization of its developmental process. The gastrula, therefore, as the bearer of important latent forces, is an entirely different thing from the gastræa, which has already 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 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 phylogenetic 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.”