earth's crust, the skeleton is generally composed of carbonate 
of lime, while in the Polycystina it is of pure glassy flint. 
The structure of the animal is in both cases very similar to 
that of the Amoeba ; but the pseudopodia are excessively long 
and filamentous, and in the case of the Foraminifera they 
largely inosculate and interlace with one another, so as to give 
rise to a regular network, while in the Polycystina they usually 
stand out like rays, without much anastomosis. In both cases, 
also, there is seen in the pseudopodia a very singular phenome- 
non, which is not known in the case of the pseudopodia of 
the Amoeba , and which has a singular interest as showing the 
highly irritable condition of undifferentiated protoplasm in 
certain instances. If, namely, we observe the pseudopodia 
of one of the Foraminifera , or Polycystina , under sufficiently 
high powers of the microscope, we discover that these ap- 
parently structureless filaments of sarcode are really largely 
made up of very minute molecules or granules, which are in 
a condition of constant movement or circulation, streaming 
out from the central mass of protoplasm, and then returning 
again into it. A similar circulation of molecules is well 
known as occurring in the protoplasm of many vegetable 
cells, and the movements, though their precise origin and 
cause are unknown, are probably really of a similar nature in 
all cases in which they have been noticed. We may conclude, 
indeed, with much probability, that this cyclosis or circulation 
of minute particles is a property of living protoplasm in one 
of the conditions in which this presents itself to our notice. 
As an example of a single cell constituting an individual 
plant, we may select the cell of Yeast plant (Saccharomyces 
cerevisice). If a minute fragment of yeast be examined micro- 
scopically under suitable conditions, it is seen to consist of a 
vast number of independent rounded cells, each of which may 
be regarded as a separate plant. Each cell — as can be well 
demonstrated by staining with carmine — consists of a central 
mass of living protoplasm, with a more or less clearly-defined 
internal nucleus, and surrounded by an apparently structure- 
less external transparent layer, or cell- wall." The cell- wall 
is composed of the characteristically but not exclusively 
vegetable substance, cellulose, and it is formed by the central 
mass of protoplasm as a kind of excretion or exudation, being 
truly in itself more or less entirely devoid of life, and consisting 
of what Dr. Beale has termed “ formed material." The yeast 
cell imbibes its own proper nutriment by endosmose through 
the porous wall, which is thinnest in the young cells and 
becomes gradually thicker with age. It has, further, the 
power of reproduction by the development of gemmae or buds 
