um was or was not a cell. It happened that, some years
before Schwann wrote, a good deal of work had been done on the minute
structure of the ovum, particularly by Purkinje and von Baer. Purkinje
in 1825[252] discovered and described in the unfertilised egg of the fowl
a small vesicle containing granular matter, which he named the
_Keimblaeschen_ or germinal vesicle. It disappeared in the fertilised
egg. As early as 1791 Poli had seen the germinal vesicle in the eggs of
molluscs, but the first adequate account was given by Purkinje. In
1827[253] von Baer discovered the true ova of mammals and cleared up a
point which had been a stumbling block ever since the days of von Graaf,
who had described as the ova the follicles now bearing his name.[254] Even
von Graaf had noticed that the early uterine eggs were smaller than the
supposed ovarian eggs; Prevost and Dumas[255] had observed the presence in
the Graafian follicle of a minute spherical body, which, however, they
hesitated to call the ovum; it was left to von Baer to elucidate the
structure of the follicle and to prove that this small sphere was indeed
the mammalian ovum. His discovery was confirmed by Sharpey and by Allen
Thomson. Von Baer found the germinal vesicle in the eggs of frogs,
snakes, molluscs, and worms, but not in the mammalian ovum; he
considered the whole mammalian ovum to be the equivalent of the germinal
vesicle of birds--a comparison rightly questioned by Purkinje (1834). In
1834 Coste[256] discovered in the ovum of the rabbit a vesicle which he
considered to be the germinal vesicle of Purkinje; he observed that it
disappeared after fertilisation. Independently of Coste, and very little
time after him, Wharton Jones[257] found the germinal vesicle in the
mammalian ovum. Valentin in 1835,[258] Wagner in 1836,[259] and Krause in
1837,[260] added considerably to the existing knowledge of the structure
of the ovum. Wagner in his _Prodromus_ called attention to the
widespread occurrence, within the germinal vesicle of a darker speck
which he called the _Keimfleck_ or germinal spot, known sometimes as
Wagner's spot. He recognised the _Keimfleck_ in the ova of many classes
of animals from mammals to polyps. Frequently more than one _Keimfleck_
occurred.

Schwann had therefore a good deal of exact knowledge to go upon in
discussing the significance of the ovum for the cell-theory. There were
two possible interpretations. Either the ovum was a cell and the
ge