THE CELL. ' 17 



§15. 



Anotlier phaenomenoii — that of reproduction — is a result of, and 

 is indissolubly connected with, tlie growth, of the cell ; for multi- 

 plication is merely the extension of growth beyond the limits of indi- 

 vidual cohesion. There are various modes of cell-multiplication j 

 the simplest of these is a direct result of growth. A bud is formed 

 by the cell-body growing out on one side. This gradually increases 

 in size,, and breaks off from the mother-cell, when it becomes a new 

 free cell. The number of young cells which are budded off from 

 a single cell is not always the same, also the part taken by the 

 nucleus of the mother-cell in the process, varies. This mode of 

 multiplication by gemmation passes imperceptibly into the more 

 common mode of multiplication by fission. Gemmation is charac- 

 terised by the difference in size which obtains at first between 

 the cells that are formed and their mother-cell. If they break 

 off at once they do not nearly equal it in size ; if they delay their 

 separation from the mother-cell they gradually get to equal it, 

 and then the products of division are almost or altogether equal to 

 one another, so that there is no possibility of distinguishing mother 

 from daughter. It is evident that in proportion to the extent to 

 which the products of division differ from one another in size, 

 does division become more and more like gemmation; the whole 

 difference therefore between fission and gemmation lies in the 

 amount of protoplasm which is given over by the parent-cell to the 

 one which arises from it. The difference is a quantitative one 

 merely. Division commences by an enlargement of the nucleus ; in 

 some cases by a formation of fresh nuclei. 



No form of reproduction other than multiplication, by fission oi* 

 by gemmation has been certainly observed in the animal-cell; a 

 large number of the modes of cell-multipKcation, which have been 

 stated to obtain, such as the so-called endogenous cell-formation and 

 similar processes, are merely forms of fission. As to free or spon- 

 taneous cell-formation, so much at least is certain, that it is not as 

 common as was once supposed. 



When the nucleus divides and the cell goes on growing without 

 the protoplasm becoming marked off into separate portions corre- 

 sponding to the nuclei, the structure which is formed cannot be 

 any longer regarded as a single cell. But it is not a compound 

 of cells either, for this would presuppose the existence of a number 

 of separate cells. This condition has therefore been very rightly 

 regarded as a special one, and called a Syncytium.. Structures 

 of this kind are found in nearly all groups of animals. The same 

 result is obtained by the Concrescence of a number of separate 

 cells, the protoplasm of which runs together into a continuous 

 mass, in which there are of course a number of nuclei. 



While the protoplasm in the above-mentioned series of pheeno- 

 mena undergoes no perceptible changes in constitution, a change 



c 



