uscs, by Remak and by
Purkinje on nerve cells, by Donne on the cells of the conjuctiva, cornea
and lens. He reports, too, that Turpin had compared the epithelial cells
of the vagina with the cell-tissue of plants. Mueller himself had not
only recognised the cellular nature of the notochord, but had observed
the cells of the vitreous humour, fat cells and pigment cells, and even
the nuclei of cartilage cells. From Schwann (1839) we learn that C. H.
Schults had followed back the corpuscles of the blood to their original
state of nucleated cells, and that Werneck had recognised cells in the
embryonic lens. A preliminary notice of Schwann's own work appeared in
1838 (Froriep's _Notizen_, No. 91, 1838), the full memoir in 1839, under
the title _Mikroskopische Untersuchungen ueber die Uebereinstimmung in
der Struktur und dem Wachstume der Tiere und Pflanzen_.[251]

Theodor Schwann was a pupil of Johannes Mueller, and we know that Mueller
took much interest in the new histology. It is probably to his influence
that we owe Schwann's brilliant work on the cell, which appeared just
after Schwann left Berlin for Loewen. Schwann was himself, as his later
work showed, more a physiologist than a morphologist; he did quite
fundamental work on enzymes, discovering and isolating the pepsin of the
gastric juice; he proved that yeast was not an inorganic precipitate but
a mass of living cells; he carried out experiments directed to show that
spontaneous generation does not occur. We shall see in his treatment of
the cell-theory clear indications of his physiological turn of mind.
Schwann was only twenty-nine when his master-work appeared, and the book
is clearly the work of a young man. It has the clear structure, the
logical finish, which the energy of youth imparts to its chosen work. So
the work of Rathke's prime, the _Anatomische-philosophische
Untersuchungen_ of 1832 shows more vigour and a more reasoned structure
than his later papers. Schwann's book is indeed a model of construction
and cumulative argument, and even for this reason alone justly deserves
to rank as a classic.

The first section of his book is devoted to a detailed study of the
structure and development of cartilage cells and of the cells of the
notochord, and to a comparison of these with plant cells. He accepts
Schleiden's account of the origin and development of nuclei and cells as
a standard of comparison; and he seeks to show that nucleus and
nucleolus, cell-wa