317] "Principes d'Embryogenie, de Zoogenie et de
Teratogenie," _Mem. Acad. Sci._, xxv., pp. 1-943, pls.
xxv., 1860.

[318] "On the Morphology of the Cephalous Mollusca,"
_Phil. Trans._, 1853, _Sci. Memoirs_, i., pp. 152-92.

[319] "Observations sur les changements de forme que les
divers Crustaces eprouvent," _Ann. Sci. nat._ (1) xxx.,
p. 360, 1833.

[320] "Considerations sur quelques principes relatifs a la
classification naturelle des animaux," _Ann. Sci. nat._
(3) i., p. 65, 1844.

[321] _Supra_, pp. 79-83. Also _Precis d'anatomie
transcendante, principes d'organogenie_, Paris, 1842.

[322] The inversion of the organs shown by Vertebrates as
compared with Invertebrates is due to the reversed
position of the embryo relatively to the yolk! (pp.
821-6).

[323] It is worth while recording that Serres enunciated a
"law of symmetry" according to which the embryo is
formed by the union of its two symmetrical halves--a law
which recalls the "concrescence theory" of His and some
modern embryologists.

[324] "Embryologie comparee du Brochet, de la Perche, et
de l'Ecrevisse," _Ann. Sci. nat._ (4), i., p. 237, 1854;
ii., p. 39, 1854. _Mem. Savans etrangers_, xvii.

[325] _Ann. Sci. nat._ (4) xvi., p. 113, 1861; xvii., p.
88, 1862; xviii., p. 5, 1862; xix., p. 5, 1863.

[326] xx., p. 5, 1863.

[327] Particularly in his _Blennius_ (1833) and _Natter_
(1839).

[328] In the "preliminary notice" of his Crayfish
paper--_Isis_, pp 1093-1100, 1825.

[329] "On the Anatomy and the Affinities of the Family of
the Medusae," _Phil. Trans._, 1849; _Sci. Memoirs_, i.,
pp. 9-32.

[330] _Phil. Trans._, cxliii., p. 368, 1853.

[331] The principle of achromatism was discovered (by
Fraunhofer) and achromatic microscopes introduced in the
early part of the 19th century. The use of chemical
reagents, such as acetic acid, and various hardening
fluids, came into fashion not long after. J. Mueller
seems to have been one of the first to realise their
importance. Remak himself invented one or two fixing and
hardening mixtures (pp. 87, 127, 1855), which enabled
him to cut excellent hand sections. Section-cutting
machines were not invented till later (V. Hensen, 1866,
His, 1870).

[332] _Untersuchungen ueber die Entwickelung der
Wirbelthiere_,