less independent living unity, whose life is not entirely
identified with the life of the plant as a whole. "Each cell," he
writes, "carries on a double life; one a quite independent and
self-contained life, the other a dependent life in so far as the cell
has become an integral part of the plant" (p. 138).

So long as the definition of the plant cell embraced little more than
the hardened cell-wall it was little wonder that "cells" in this sense
were not recognised in animal tissues, except in a few exceptional
cases--as in the notochord by Johannes Mueller.[242] Careful observation of
animal tissues discovered in some cases the existence of discontinuous
units of structure, but these were not, as a rule, recognised before
1838 as analogous to plant cells. Von Baer, for example, observed that
the young chick embryo was composed partly of an albuminous mass and
partly of _Kuegelchen_ or little globules suspended in it
(_Entwickelungsgeschichte_, i., pp. 19, 144). Since such _Kuegelchen_
disposed in a row formed the notochord (i., p. 145) it seems probable
that his _Kuegelchen_ were really cells. Similarly A. de Quatrefages[243]
in 1834 saw and figured segmentation spheres in the developing egg of
_Limnaea_, but he called them globules and did not recognise their
analogy with the cells of plants. According to M'Kendrick,[244] Fontana,
so far back as 1781,[245] described cells with nuclei in various tissues,
and used acids and alkalis to bring out their structure more clearly.
But it was not till 1836-7-8 that a fairly widespread occurrence of
cells in animal tissues was recognised. The pioneer in this seems to
have been Purkinje, who described cells in the choroidal plexus in
1836,[246] and compared gland cells with the cells of plants in 1837.[247]
Henle in 1837[248] and 1838[249] described various kinds of epithelial
tissue, distinguishing them according to the kind of cell composing
them; he also discovered the mode of growth of stratified epithelium.
Valentin[250] appears to have seen cells in cartilage and epithelium even
before Henle, and to have observed cells in the blastoderm of the chick.
In his report on the progress of anatomy during 1838 Johannes Mueller was
able to refer to quite a number of papers dealing with the occurrence of
cells in animal tissues. In addition to those already noted, he mentions
work by Breschet and Gluge on the cells of the umbilical cord, by
Dumortier on the cells in the liver of moll