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 in 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 like 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 again. 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 following 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 la}-er of the ger- 

 minal membrane of the bird's egg (p. 60), and some cells of 

 the crystalline lens (p. 88). PI. I, fig. 10, c, represents one 



