has led to the assertion that the latter are permanent embryos of the former. The embryo of a Vertebrate shows the vertebrate type from the very beginning, and retains this type throughout the whole course of its development; it never is, and never can be, either a Mollusc or an Articulate" (xx., p. 54). "We are led to establish ... as the general result of our researches, the existence of several types, and, consequently, of different plans, in the development of animals. These different types are manifested from the very beginning of embryonic life; the characters distinguishing them are therefore primordial, and we can say with M. Milne-Edwards that _everything goes to prove that the distinction established by Nature between animals belonging to different phyla is a primordial distinction_" (p. 58). In other directions also von Baer's work was confirmed and extended by later observers--those parts of it particularly that had reference to the germ-layer theory, and to the concept of histological differentiation. His germ-layer theory was accepted in its main lines by Rathke, Bischoff and Lereboullet, and applied by them to the multitude of new facts they discovered. Rathke, in particular, was a firm upholder of the doctrine, and made considerable use of it in his writings.[327] Even before the publication of von Baer's book he had interpreted in terms of the germ-layer theory sketched by his friend Pander the splitting of the blastoderm which occurs in the early development of _Astacus_, whereby there are formed a serous and a mucous layer, one inside the other--like the coats of an onion, to use his own expressive phrase.[328]