formations disappear after fertilization without a trace or 

 whether they stand in continuous genetic connection with 

 the nucleus (and the nucleolus) of the cells of the embryo. 

 On the foundation of a one-sided and primitive understanding 

 of the cellular theory, fantastic presentations sometimes 

 grew, like the ideas of Reichert, which resurrected the 

 long-buried theory of preformation. 



For the purposeful coordination of the efforts of 

 zoologists studying invertebrate development, the theory of 

 embryonic layers, which still had not become a broad 

 scientific generalization, could not serve. Its correctness 

 was proven only for vertebrates, but the application of 

 the theory of embryonic layers to invertebrates was believed 

 by nearly no one. Rathke's old data concerning crayfish were 

 known. More than a quarter of a century later Zaddach also 

 reported on the embryonic layers of insects, admitting to 

 his descriptions crude morphological mistakes. 



The data related to the development of different types 

 of invertebrates were accumulated relatively slowly, because 

 a theoretical conception for which this material could be 

 used did not exist. The study of the types of structure and 

 development of animals, taking its beginning from the opinion 

 of Cuvier and Baer, fell into decay. The formally opposite 

 theory of types, that of Naturphilosophie, the idea of 

 unity of planes, caused little enthusiasm. Only after fifty 

 years, after the appearance of Darwin's theory, did this 

 period of mental stagnation end. Then the naturalists were 

 divided into two camps — the hearty supporters and the violent 

 opponents of the theory of evolution. Practically none of the 

 contemporaries of the great reformer of biological science 

 could keep an olympian calm and sustain neutrality for any 

 length of time. 



The investigations preceding the publications of the 

 ORIGIN OF SPECIES played, however, its historical role. 

 Factual material was collected which Darwin and his early 

 followers used. In this period came the important works of 

 the Russian embryologists A. Grube, A. D. Nordmann, 

 N. A. Warnek, and A. Krohn. 



To Dorpat University professor A. Grube belonged serious 

 investigation on the development of annelids; he studied 

 the embryonic and partly post -embryonic development of the 



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