rminal vesicle its nucleus, or else the germinal vesicle was itself a
cell within the larger cell of the ovum and the germinal spot was its
nucleus. Schwann had some difficulty in deciding which of these views to
adopt, but he finally inclined to the view that the ovum is a cell and
the germinal vesicle its nucleus, basing his opinion largely upon
observations by Wagner which tended to prove that the germinal vesicle
was formed first and the ovum subsequently formed round it. But the ovum
was not, in Schwann's view, a simple cell, for within it were contained
yolk-granules, one set apparently containing a nucleus, the others not.
Even the second set, those composing the yellow yolk, were considered by
Schwann to deserve the name of cells, because, although a nucleus could
not be observed in them, they had a definite membrane, distinct from
their contents--a conception of the cell obviously dating from the
earliest botanical notions of cells as little sacs. The yolk cells were
not mere dead food material but living units which took part in the
subsequent development of the egg. The relation between the unfertilised
egg and the blastoderm which arises from it is not made altogether clear
by Schwann. According to his account the cells of the blastoderm are
formed actually in the ovum. Round the nucleus of the egg appears a
_Niederschlag_ or precipitate which is the rudiment of the blastoderm
(p. 68). When the egg leaves the ovary the nucleus disappears, leaving
behind it this rudiment of the blastoderm, which rapidly grows and
increases in size. The blastoderm of the chick before incubation is
found to be composed of spherical anucleate bodies which Schwann
considers to be cells, because they almost certainly develop into the
cells of the incubated blastoderm, which are clearly recognisable as
such after eight hours' incubation. The serous and mucous layers can be
distinguished after sixteen hours' incubation, and it is found that the
cells of the serous layer contain definite nuclei, though such seem to
be absent in the cells of the mucous layer. Between the two layers other
cells are formed belonging to the vessel layer, which is, however, in
Schwann's opinion not a very definitely individualised layer.

Schwann's next step is a detailed demonstration of the origin of each
tissue from simple cells such as those composing the incubated
blastoderm.

"The foregoing investigation has taught us that the whole ovum shows
noth