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]

An ingenious application of the Pander-Baer theory was made by Huxley,
who compared the outer and inner cell-layers which form the groundwork
of the Coelentera with the serous and mucous layers of the vertebrate
germ.[329] He laid stress, it is true, rather on the physiological than on
the morphological resemblance. "A complete identity of structure," he
writes, "connects the 'foundation membranes' of the Medusae with the
corresponding organs in the rest of the series; and it is curious to
remark, that throughout, the outer and inner membranes appear to bear
the same physiological relation to one another as do the serous and
mucous layers of the germ; the outer becoming developed into the
muscular system, and giving rise to the organs of offence and defence;
the inner, on the other hand, appearing to be more closely subservient
to the purposes of nutrition and generation" (p. 24). Von Baer had
already hinted at this homology in the second volume of his
_Entwickelungsgeschichte_ (1837), where he s