DIRECT AND INDIRECT DIVISION OF NUCLEI. 183 



derived from the nucleus of the cell, although the exact manner of its formation is 

 not clear. Important functions in connection with the nutrition and functions of 

 the cell generally have been ascribed to the paranuclei, but further investiga- 

 tions are needed in order to show how far their occurrence is general, and what is 

 the precise nature of the part they play in the economy of the cell. 



The general question whether the nucleus of a cell has any functions 

 beyond those concerned with the reproduction and division of the cell also 

 requires further elucidation. The existence of animal cells or organisms without 

 nuclei has been affirmed by various authors, but in all but the lowest animals and 

 plants every cell is undoubtedly nucleated. Attempts have been made with more or 

 less success to separate off portions of cell-protoplasm without nuclei, and to keep 

 them for some time under observation with the microscope. It is stated that under 

 favourable circumstances the separated protoplasm will continue for a while to live, 

 and will exhibit amoeboid movements, but that it does not grow nor form a cell-mem- 

 brane (vegetable cells), although the part of the cell which still contains the nucleus 

 enlarges to the original size of the whole cell, and does form a new cell-membrane. 

 In the case of the amoeba it has been found that the portion deprived of a nucleus 

 will take in food-particles as before, but does not digest them (Hofer), nor are its 

 amoeboid changes so pronounced as in the part which still has the nucleus. The 

 action of the contractile vacuoles continues, however, unaltered. From this it has 

 been inferred that the presence of the nucleus is necessary to the growth and 

 nutrition of protoplasm, and that it may therefore be concerned to a certain extent 

 in regulating the anabolic processes of the cell as well as its division and reproduc- 

 tion. In the ovum of Dytiscus, Korschelt has observed an intimate relationship 

 between the formation of food-granules within the ovum and the protrusion of pseudo- 

 podium-like processes from the nucleus, and infers from these observations that the 

 nucleus may be directly concerned in the assimilation of nutriment. But whatever 

 other functions it may possess there is no doubt that the most obvious function of 

 the nucleus is connected with the division and multiplication of the cell. 



THE DIVISION AND MULTIPLICATION OP CELLS AND NUCLEI. 



Direct and indirect division of nuclei. According to the scheme of cell- 

 division which was formulated by Remak, the division of the cell is preceded by the 

 division of the nucleus, which forms by a simple process of constriction two equal 

 parts, and this again is initiated by the previous division of the nucleolus. The 

 essence of this scheme consists in the simple fission of the nucleus, which directly 

 divides to form two smaller or daughter nuclei : it is commonly spoken of as the 

 process of " direct " division. 



The term " indirect " division has been given on the other hand to a process which 

 is of very wide and almost universal occurrence in both animal and plant cells, in 

 which the chromoplasm undergoes previously to the fission of the nucleus a series of 

 remarkable changes, the ultimate result of which is the separation of its chromatin 

 into two exactly similar portions, which go respectively to form the chromatin of the 

 daughter nuclei. Concomitantly with these changes in the nucleus the protoplasm 

 of the cell also exhibits active changes, which ultimately result in its accumulation 

 around each of the daughter-nuclei and the formation of two separate cells. To the 

 changes which are undergone by the nucleus in this "indirect" process of division 

 the name JcaryoJcinesis was given by Schleicher : the appearances which the nuclei 

 present at different stages of karyokinesis are spoken of as mitoses l (Flemming). 



1 fjitrot, a thread. The indirect process of division is also termed " mitotic," and the direct is known 

 in contradistinction as " amitotic. " 



