380 M. Coste on the Formation of Cells. 
tible of playing a more or less active part in the organization of 
plants. But this mode of formation of the walls of the cells not 
being the only one observed by M. Mirbel, this physiologist has 
been induced to admit, that in vegetables nature attains her ob- 
ject by different means. 
However, this manner of viewing and judging of the phzno- 
mena of which the cambium is the seat was soon accompanied by a 
diametrically opposite system, the exclusive foundation of which 
does not admit of the possibility of an exception. This system, 
contrived by Schleiden to explain the formation of the vegetable 
tissue, and applied by Schwann to the organization of animals, as 
we shall presently see, is essentially no more than a generalization 
a priort of Purkinje’s theory of the development of the egg in 
the ovary,—a theory, a large part of which has unfortunately lost 
much of its value from new discoveries which have diminished 
its importance, or even reduced it to the level of the most rare 
exceptions. 
Purkinje, after havmg recognised that the germinal vesicle, 
among all the component parts of the bird’s egg, was that which 
from its origin had a proportionately more considerable develop- 
ment, supposed that it was first formed, and considered it as a 
centre around which were successively deposited, first the vitellus, 
and then the vitelline membrane, which, in its turn, coagulated 
at the periphery of the yolk to complete the ovarian egg, and to 
inclose its elements in an enveloping membrane. This successive 
union of concentric parts, mechanically superadded around each 
other, so that the most external are the most recent, having ap- 
peared to Schleiden and Schwann the most simple means of con- 
ceiving the formation of the vesicular walls, these naturalists 
formed it into a general theory of the development of the cell; 
and with them the enunciation of the special fact, modified as we 
shall presently show, has become the foundation of a universal 
principle. 
Consequently they admitted, that in the diffuse and structure- 
less homogeneous substance, the cytoblastema, by means of a 
concentration of this substance, corpuscles were formed; these 
were so extremely minute, that even the highest powers of the 
microscope did not allow of their always being detected. These 
corpuscles, called nucleoli, are so many centres around each of 
which a layer of finely-granular matter is deposited, which is not 
at first distinctly limited in its circumference, but which finally 
becomes more clearly outlmed, and forms more or less regularly 
spheroidal, elliptical or lenticular agglomerations. 
Each of these minute accumulations of amorphous matter 
around one or even several nucleoli, which they unite, is called 
a cytoblast or nucleus, and forms the second phase of the process 
