172 SURVEY OF CELL-LIFE. 
impregnated with the nutritive material for the other kind of 
cells. 
According to Schleiden, new cells are never formed in the 
intercellular substance in plants ; in animals, on the contrary, 
a generation of cells within cells is the less frequent mode, but 
this does occur, and in such a way, that a threefold or four- 
fold generation may take place im succession within one cell. 
Thus, according to R. Wagner’s observations (see the Supple- 
ment), the Graafian vesicle appears to be an elementary cell ; 
the ovum is developed within it in ike manner as an element- 
ary cell; within this, again, according at least to observations 
made upon the bird’s egg, cells are generated, some of which 
contain young cells. It appears also, that a formation of 
true cartilage-cells can sometimes take place within those 
which already exist, and that young cells (fat-cells?) may 
be generated within them agai. Several such examples 
might be brought forward; but by far the greater portion 
of the cells of cartilage are formed in the cytoblastema on 
the outside of the cells already present, and we never meet 
with a generation of cells within cells in the case of fibre, 
muscle, or nerve. 
General phenomena of the formation of cells. Round 
corpuscles make their appearance after a certain time in the 
cytoblastema which, in the first instance, is structure- 
less or minutely granulous. These bodies may either be 
cells in their earliest condition (and some may be recognized 
even at this stage), that is, hollow vesicles furnished with a 
peculiar structureless wall, cells without nuclei, or they may 
be cell-nuclei or the rudiments of cell-nuclei, round which cells 
will afterwards be formed. 
The cells without nuclei, or, more correctly, the cells in 
which no nuclei have as yet been observed, occur only 
in the lower plants, and are also rare in animals, For the 
present, however, the followmg must be regarded as such, 
viz.: the young cells contained within others in the chorda 
dorsalis (see p. 13), the cells of the yelk-substance in the 
bird’s egg (p. 50), the cells in the mucous layer of the ger- 
minal membrane of the bird’s egg (p. 60), and some cells of 
the crystalline lens (p.88). Pl. I, fig. 10, c¢, represents one 
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a 
