36 
THE ORGANIC CELL 
tissues the cell- walls are connected by intercellular bridges. 
Bridges of a similar nature have been demonstrated with 
certainty in practically all forms of epithelium, also in con- 
nective tissue cells and nerve cells. Retzius and others have 
shown that the cells of the Graafian follicles of the ovary are 
not only connected with one another by bridges, but are also 
connected with the ovum. 
As a result of this evidence many recent observers have 
accepted Heitzmann’s theory. 
It is probably a little premature to accept this hypothesis 
in full in regard to the adult, though in the embryonic stages 
there seems to be no doubt as to the general continuity between 
cells. 
Sedgwick has shown that in the vertebrates the embryonic 
body in its earlier stages is a continuous reticulum, and E. B. 
Wilson points out ‘ that in a total cleavage, such as that of 
Amphioxus , the results of experiment on the early stages of 
cleavage are difficult to explain, save under the assumption 
that there must be a structural continuity from cell to cell that 
is broken by mechanical displacement of the blastomeres.’ 
Mrs. Andrews maintains that during the cleavage of 
Echinoderm eggs the blastomeres spin protoplasmic threads by 
which continuity is established between them after each 
division. (See ‘ Filose Activities in Metazoan Eggs,’ Zool. 
Bull. II. 1, also ‘ Activities of Polar Bodies,’ Arch. Entom. VI. 
2.) Flemming has demonstrated that when white corpuscles 
move among epithelial cells the bridges become broken, but are 
re-formed afterwards. 
The absolute function of the cell- bridges is at present not 
definitely known. That they are not merely channels for the 
passage of nutrition, but form the roads by which physiological 
impulses are transmitted, is proved by Townsend’s experiments 
on plants. Townsend shows that in root-hairs and pollen tubes, 
if the protoplasm is broken, a membrane may be formed by 
both nucleated and non-nucleated fragments — by the latter 
however * only when they remain connected with the nucleated 
masses by protoplasmic strands, however fine.’ 
Should these connecting threads get broken, the power of 
