THE CELL IN RELATION TO THE MULTICELLULAR BODY 43 



bridges not only with one another, but also with the ovum, a conclu- 

 sion which, if established by further research, will be of the greatest 

 interest. 



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 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 arthro- 

 pods, 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 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, jus- 

 tify any general statement regarding the occurrence, origin, or physi- 

 ological meaning of the protoplasmic continuity of cells ; and a most 

 important field here lies open for future investigation. 



LITERATURE. I 1 



Altmann, R. Die Elementarorganismen und ihre Beziehungen zu den Zellen, 2d 



ed. Leipzig, 1894. 



Van Beneden, E. (See Lists II., IV.) 

 Boveri, Th. (See Lists IV., V.) 

 Butschli, 0. Untersuchungen liber mikroskopische Schaume und das Protoplasma. 



Leipzig (Engelmann), 1892. 

 Engelmann, T. W. Zur Anatomic und Physiologic der Flimmerzellen : Arch. ges. 



Phys., XXIII. 1880. 



von Erlanger, R. Neuere Ansichten Uber die Struktur des Protoplasmas : Zool. 

 \\.^<). 1896. 



1 See also Introductory list, p. 12. 



