In the last paragraph of his work, von Baer reaches out from his law to the cosmos:

If our most general result is true, then there is one fundamental thought [Grundgedanke] that permeates all the forms and stages of animal development and governs all their relationships. It is the same thought that, in the cosmos [Weltraume], collects the separated masses into spheres and binds these together into a solar system; the same that allows the scattered dust on the surface of the metallic planet to develop into living forms. This thought, however, is nothing but life itself, and the words and syllables in which it expresses itself, are the different forms of the living. (pp. 263–264)

Von Baer attacked recapitulation from both sides of his thought. On the one hand, the functional perspective of Cuvier would not permit the unilinear classification that recapitulation required. Ironically though, von Baer's most effective argument lay in his particular version of a general principle that he shared with the Naturphilosophen—a principle that validated recapitulation for Oken and Meckel. Oken, Meckel, and von Baer all agreed that a single developmental tendency pervaded nature. For Oken, it was the progressive addition of organs or powers; for Meckel, the coordination and specialization of parts. Both yield recapitulation. For Oken, the human embryo begins in the primal chaos of zero and adds organs in a sequence reflecting the order of lower adults. For Meckel, the human embryo begins with uncoordinated parts and develops an integrated set of specialized organs in a sequence running parallel with the ascending series of lower adults. But for von Baer, the single tendency is differentiation, the development of the special from the general. This precludes recapitulation. The human embryo begins as a generalized vertebrate retaining the potential to become any species of its type; it cannot represent the completed adult of any lower animal.

Louis Agassiz and the Threefold Parallelism

Von Baer was not the only great biologist caught in a dilemma of allegiance to the contrasting schools of Naturphilosophie and Cuvierian functionalism. Von Baer's dilemma was particularly acute, for he held both viewpoints concurrently. Louis Agassiz espoused them sequentially. As a young man, he adopted many concepts of romantic biology, notably recapitulation. Later, when he had abandoned the easy explanations that Naturphilosophie provided for recapitulation, he had to supply new justifications consistent with the spirit of Cuvier's thought.

During the 1820s, Agassiz studied with several of Germany's leading Naturphilosophen. Lurie (1960) supposes that Tiedemann