embryo of higher animals with the adult of
lower was widely spread at this time among German zoologists. We find,
for example, in Tiedemann's brilliant little textbook[146] the statement
that "Every animal, before reaching its full development, passes through
the stage of organisation of one or more classes lower in the scale, or,
every animal begins its metamorphosis with the simplest organisation"
(p. 57).

Thus the higher animals begin life as a kind of fluid animal jelly which
resembles the substance of a polyp; the young mammal, like the lower
Vertebrates, has only a simple circulation, and, like them, lives in
water (the amniotic fluid); the frog is first like a worm, then develops
gills and becomes like a fish (p. 57). In his work on the anatomy of the
brain,[147] Tiedemann established the homology of the optic lobes in birds
by comparing them with foetal corpora quadrigemina in man (see Serres,
_Ann. Sci. nat._, xii., p. 112).

J. F. Meckel, in 1811, devoted a long essay to a detailed proof of the
parallelism between the embryonic states of the higher animals and the
permanent states of the lower animals. In a previous memoir in the same
collection[148] (i., 1, 1808) he had made some comparisons of this kind in
dealing with the development of the human foetus; in this memoir (ii.,
1, 1811) he brings together all the facts which seem to prove the
parallelism.

His collection of facts is a very heterogeneous one; he mingles
morphological with physiological analogies, and makes the most
far-fetched comparisons between organs belonging to animals of the most
diverse groups. He compares, for instance, the placenta with the gills
of fish, of molluscs and of worms, homologising the cotyledons with the
separate tufts of gills in _Tethys, Scyllaea_ and _Arenicola_(p. 26).
This is purely a physiological analogy. He compares the closed anus of
the early human embryo with the permanent absence of an anus in
Coelentera, and the embryo's lack of teeth with the absence of teeth in
many reptiles and fish, in birds, and in many Cetacea (p. 46).[149] These
are merely chance resemblances of no morphological importance. He
considers bladderworms as animals which have never escaped from their
amnion, and _Volvox_ as not having developed beyond the level of an egg
(p. 7). He lays much stress upon likeness of shape and of relative size,
comparing, for instance, the large multilobate liver of the human foetus
with the many-lobed li