ELEMENTARY VITAL PHENOMENA 191 



duction, there occurs asexual increase, by fission and gemmation. 

 In both cases whole complexes of cells are separated off. In 

 fission, e.g., in certain worms (Fig. 69), the whole 

 body, after having reached a certain size by 

 cell-division, is constricted into two or more parts 

 which regenerate themselves again into complete 

 individuals. In gemmation, e.g., in many coelente- 

 rates (Fig. 70), there is formed in one part of 

 the body by rapid cell-multiplication a bud, 

 which contains cells from the essential body- 

 layers and likewise becomes constricted off to 

 regenerate into a new individual. 



In all cases, therefore, reproduction, whether 

 asexual or sexual, takes place by cell-division 

 alone, and this depends upon growth. We will ^'uon-^l%l^t 

 now follow the different kinds of cell-division (After ciaus.) 

 somewhat more in detail and consider the 

 remarkable phenomena that take place in the cell. 



2. The Forms of Cell-division 



In order that the daughter-cells of a cell-division may be 

 capable of life, both the nucleus and protoplasm, as already remarked, 

 must divide. But while the division of the protoplasm is very 

 simple, the cell-body simply becoming constricted deeper and 

 deeper by a groove until the protoplasm is separated into two 

 halves, in most cases there appear in the nucleus extremely 

 complicated changes, which in most cells, both animal and 

 plant, agree remarkably in essentials. Regarding the more 

 minute phenomena of cell-division a literature so large as to be 

 almost beyond mastery has appeared during the last two de- 

 cades, since investigators, misled by the very peculiar behaviour 

 of the nucleus in cell-division, adopted the erroneous view that 

 the nucleus is the sole essential cell-constituent and must be 

 studied as exhaustively as possible in its " active ' condition. 

 The fundamental investigations of the phenomena of cell-division 

 comprise the admirable ones of Biitschli (76), Flemming ('82), 

 Strasburger, ('80, '88), 0. Hertwig ('76, '77, '78, '92), van Beneden 

 ('87), Boveri ('87, '88, '90), and others, who have found objects 

 best fitted for this purpose in the cells of young larvse of sala- 

 manders, in the pollen-cells of lilies, and in the transparent 

 eggs of the sea-urchin and the round- worm of the horse. 



a. Direct Cell-division 



The simplest form of cell-division is the direct or amitotic cell- 

 division, which, however, is comparatively rare and, beyond certain 



