This interesting view, long received with scepticism, has been in a measure sustained by later researches, though it still remains sub judice. Tangl, Gardiner, and many later observers have shown that the cell-walls of many plant-tissues are traversed by delicate intercellular bridges, and similar bridges have been conclusively demonstrated by Bizzozero, Retzius, Flemming, Pfitzner, and many others in the case of animal epithelial cells (Figs. 1, 9). The same has been asserted to be the case with the smooth muscle-fibres, with cartilage-cells and connective-tissue cells, and in a few cases with nervecells. Paladino and Retzius ('89) have endeavoured to show, further, that the follicle-cells of the ovary are connected by protoplasmic bridges not only with one another, but also with the ovum, a conclusion which, if established by further research, will be of the greatest interest.
As far as adult animal-tissues are concerned, it still remains undetermined how far the cells are in direct protoplasmic continuity. It is obvious that no such continuity exists in the case of the corpuscles of blood and lymph and the wandering leucocytes and pigment-cells. In case of the nervous system, which from an a priori point of view would seem to be above all others the structure in which protoplasmic continuity is to be expected, the latest researches are rendering it more and more probable that no such continuity exists, and that nerve-impulses are transmitted from cell to cell by contact-action. When, however, we turn to the embryonic stages we find strong reason for the belief that a material continuity between cells must exist. This is certainly the case in the early stages of many arthropods, where the whole embryo is at first an unmistakable syncytium ; and Adam Sedgwick has endeavoured to show that in Per/pa/us and even in the vertebrates the entire embryonic body, up to a late stage, is a continuous syncytium. I have pointed out ('93) that even in a total cleavage, such as that of Amphiarus or the echinoderms, 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. This conclusion is supported by the recent work of Hammar ('96), whose observations on sea-urchin eggs I can in the main confirm.
As the subject now lies, however, the facts do not, I believe, justify any general statement regarding the occurrence, origin, or physiological meaning of the protoplasmic continuity of cells ; and a most important field here lies open for future investigation.