60 GENERAL SKETCH 'OF THE CELL 



are traversed by delicate intercellular bridges. Similar bridges have 

 been conclusively demonstrated by Ranvier, Bizzozero, Retzius, Flern- 

 ming, Pfitzner, and many later observers in nearly all forms of epithe- 

 lium (Fig. i ) ; and they are asserted to occur in the smooth muscle-fibres, 

 in cartilage-cells and connective tissue-cells, and in some nerve- 

 cells. Dendy ( '88), Paladino ( '90), and Retzius ( '89) have endeav- 

 oured 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 ; and similar protoplasmic bridges between germ-cells 

 and somatic cells have been also demonstrated in a number of plants, 

 e.g. by Goroschankin ( '83) and Ikeno ( '98) in the cycads and by A. 

 Meyer ( '6) in Volvox. On the strength of these observations some 

 recent writers have not hesitated to accept the probability of Heitz- 

 mann's original conception, A. Meyer, for example, expressing the 

 opinion that both the plant and the animal individual are continuous 

 masses of protoplasm, in which the cytoplasmic substance forms a 

 morphological unit, whether in the form of a single cell, a multi- 

 nucleated cell, or a system of cells. 1 Captivating as this hypothesis 

 is, its full acceptance at present would certainly be premature ; and 

 as far as adult animal tissues are concerned, it still remains unde- 

 termined 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 that in which protoplasmic con- 

 tinuity is to be expected, its occurrence and significance are still a 

 subject of debate. When, however, we turn to the embryonic stages 

 we find strong reason for the belief that a material continuity between 

 cells here exists. 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 

 Peripatus 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 Amphioxus or the echi- 

 noderms, 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 mechan- 

 ical displacement of the blastomeres. This conclusion is supported 

 by the recent work of Hammar ( '96, '97), whose observations on 

 sea-urchin eggs I can in the main confirm. 



Among the most interesting observations in this direction are 

 those of Mrs. Andrews ('97), 2 who asserts that during the cleavage 



1 '96, p. 212. Cf. also the views of Hanstein, Strasburger, Russow, and others there 

 cited. 2 Cf. also E. A. Andrews, '98, I, '98, 2. 



