72 GENERAL PHYSIOLOGY 



as cartilage (Fig. 12). Many epithelial cells (Fig. 13, a) contain 

 more than two nuclei ; and the large ciliate infusorian, Opalina 

 (Fig. 13, b), which lives parasitically in the intestine of the frog, 

 contains a considerably larger number. Forms with innumerable 

 nuclei are to be found among the marine algae : e.g., in the 

 thin lamellar protoplasmic layer of Caulerpa (Fig. 14), a giant 

 cell of the shape and size of a leaf, there lies an immense number 

 of nuclei, all of which together with the protoplasm are moving in 

 a constant slow stream between the cell-walls, i.e., the two surfaces 

 of the leaf. 



All these organisms containing several nuclei can be separated 

 as multinucleate cells from multicellular tissues by the fact that 

 in the former the protoplasmic territory immediately surrounding 

 the individual nuclei is not sharply defined from the neighbouring 

 protoplasm, but together with all the rest of the protoplasm con- 

 stitutes a unitary mass which appears as a whole shut off from the 



FIG. 13. a, Epithelium-cell, containing several nuclei from the urinary bladder of man. (After 

 Virchow.) b, Opalina ranarum, a unicellular ciliate infusorian, containing many nuclei, from 

 the intestine of a frog. (After Zeller.) 



outside by a definite surface, while in the tissue every individual 

 protoplasmic territory which belongs to a nucleus is sharply 

 separated from all the rest. The multinucleate cell, therefore, 

 represents one cell, which is characterised as a whole by a definite 

 form of surface ; the tissue, however, consists of a sum of single 

 cells, each one of which has its own sharply defined form. 



The distinction between multinucleate cells and genuine tissues 

 becomes more difficult in the case of certain low organisms, the 

 Myxomycetes, which have frequently been claimed by the botanists 

 as plants and by the zoologists as animals, and which in many 

 respects are of great interest. They are sometimes seen in leafy 

 forests, upon mouldy leaves or decaying tree-trunks, as white, 

 yellow, or brownish-red networks ; they often spread themselves 

 out for several decimetres upon objects by means of their delicate 

 arborescent strands (Fig. 15, /). Detailed examination shows that 

 these networks, which sometimes form thicker, lumpy masses of 

 the same appearance, are of a soft slimy consistency. If such a 



