CELL DIVISION 317 



daughter cell often divides before it is lost in the granular pro- 

 toplasm of the resting cell. In many cases the centrosome or 

 centrosomes can be found in the resting cells. In other cases, 

 division of the centrosome occurs just previous to cell division. 



690. Spindle Fibres. — The spindle fibres are achromatic sub- 

 stances, i. e., they do not stain like the chromatin. They are 

 probably derived in large part from the linin network of the 

 nucleus. The astral figures and, perhaps, the spindle in part, 

 are cytoplasmic. 



691. Resting Nucleus. — Ordinarily, after division, the cells 

 remain quiescent for a time; that is, so far as any apparent 

 changes in the nucleus are concerned. This is called the 

 resting state, although growth and other processes may be 

 actively going or. 



692. Conjugation. — Another very important process in 

 which the nucleus is especially concerned is the reverse of cell 

 division. Cell division is ordinarily followed by a period of 

 growth, then another cell division and another period of growth. 

 This constitutes the ordinary daily routine of cell life. At 

 comparatively long intervals this chain of events is broken by the 

 fusion of one cell with another. This may take place in a great 

 variety of ways, which, however, are apparently fundamentally 

 the same, and the significance of the process may best be 

 explained by the description of a number of examples. 



693. Mougeota is a filamentous alga, which consists of a 

 series of similar cells adhering to each other in a row. At 

 certain times two such filaments, lying side by side, become con- 

 nected by a series of tubes in such a way that each cell of one 

 filament is connected with a similar cell in the other filament. 

 Then the protoplasm of each cell flows into the tube where the 

 two masses unite into a single body. In some other algai swarm 

 spores are formed and set free in the water. Each 

 spore is pear-shaped and has two flagella by which it actively 

 swims about. They are all of the same size and cannot be 



