derived from tlie central protoplasm, and ultimately detached 
to form, new individuals. 
Owing to the comparative density and rigidity of its cell- 
wall, the protoplasm of the yeast plant does not give rise 
to outward processes, or “ pseudopodia,” though it may, as 
above observed, give forth external buds. For the same 
reason, the protoplasm of vegetable cells in general is usually 
devoid of any power of thrusting out pseudopodia. Amoeboid 
and moving filaments of protoplasm have, however, been 
shown by Mr. Francis Darwin* to be emitted from the cells of 
the glandular hairs of the Common Teasel ( Dijisacus sylvestrin), 
and to differ in no essential respect from the “ pseudopodia ” 
of the Rhizopods. Moreover, it is well known that the proto- 
plasm in the interior of many vegetable cells, though confined 
by an unyielding envelope, is capable of “rotation” as a 
whole, or of exhibiting a circulation of granules similar to 
that seen in the pseudopodia of the Foraminifera. Lastly, 
in certain of the life-stages of some of the lower plants 
( Myxomycetes ) we really meet with wall-less masses of proto- 
plasm, which are capable of thrusting out pseudopodia, and 
are in all essential respects morphologically similar to Amochce. 
In the case of the higher and more complex plants, as also 
in the more highly-organised animals, the ovum, or earliest 
rudiment of the future organism, has invariably the form of a 
single simple cell, the essential part of wdiich is a central mass 
of protoplasm. In these cases, however, the unicellular struc- 
ture is soon lost ; new cells are produced in larger or smaller 
numbers by processes well known to physiologists ; some of 
these cells may undergo considerable secondary modifications, 
and ultimately the organism comes to consist of an aggregate 
of cells, all of wdiich may more or less entirely retain their 
primitive form, or many of which may be highly specialised 
and developed into various complex tissues. It should also be 
noted that in addition to the tissues, as ordinarily understood, 
the body of an animal may contain a great number of cells 
which float freely in a fluid (the blood), and are not in any direct 
connection with one another. Lastly, while the cells of the 
higher animals, as a rule, resemble those of the great majority 
of plants in having firm walls, which allow of no changes 
of form, nor of outward protrusion of protoplasmic filaments, 
there are cases, even amongst the highest animals, in which 
we meet with wall-less masses of protoplasm, which contain 
distinct nuclei, and are capable of manifold mutations of shape. 
The pigment cells of the skin of the frog, of cuttle-fishes, and of 
* Quart. Journ. Microscop. Science, 1877, p. 245. 
