CELLS. 47 
cells, and it seems likely that they are present in most 
animal cells, at least in those which retain the power of 
division. 
(d) As to the cell wall, it seemed 
of much moment to the earlier 
histologists, who often spoke of 
cells as little bags or boxes. It 
is, however, the least important 
part of the cell. In plant cells 
there is usually a very distinct 
wall, consisting of cellulose. This 1G. 23.—Fertilised ovum of 
is a product, not a part, of Ascarts.—After Boveri. 
chr., Chromatin elements, two 
the protoplasm, though some from ovum nucleus and two 
protoplasm may be intimately ee Bielend ) Gee 
Fy . ‘* . rom whic 
associated with it as long as its “archoplasmic” threads 
growth continues. In animal dadinte, wartly Sods elivom: 
a me: 
cells there is rarely a very distinct 
wall chemically distinguishable from the living matter 
itself. But the margin is often different from the in- 
terior, and a slight wall may be formed by a superficial 
compacting of the threads of the cell network, or by a 
physical alteration of the cell substance, comparable to the 
formation of a skin on cooling 
porridge. In other cases, especi- 
ally in cells which are not very 
active, such as ova and encysted 
Protozoa, a more definite sheath - 
is formed around the cell sub- 
stance. Again, animal cells may 
secrete a superficial “cuticle,” e.g. 
the chitin formed by the ectoderm 
cells in Insects, Crustaceans, and 
Fic. 24.—Diagram of cell other Arthropods. 
division.—After Boveri. In animals, as well as in plants, 
chr. Chromosomes forming adjacent cells are often linked 
an cquatorial' plate; ¢s by intercellular bridges of living 
matter, which may be paths for 
the passage of materials or of disturbances from cell to cell. 
In many cases, ¢.g. of gelatinous tissue, a matrix arises out- 
side of and between the cells, as an exoplasmic product. 
In regard to cell division, the most important facts are the 
