MITOSIS (KARYOKINESIS) 21 



width until it has comi)h'ted the separation of the two 

 protopUismic masses, this being tlie commoner way in 

 the lower plants, or the wall may be produced sim- 

 ultaneously at all points at the plane of separation be- 

 tween the two protoplasts, as is the case in most higher 

 plants. In some of the lower plants the whole wall be- 

 gins to constrict at the middle, giving the appearance of 

 pinching the cell into two separate cells which are then 

 free from one another. A peculiar type of fission is that 

 termed budding, in which a small outgrowth appears at a 

 point on the cell, gradually enlarging until it is as large 

 as the old cell and then l^ecoming separated from it by 

 constriction of the wall at the point of emergence. This 

 is especially characteristic of, but not confined to, some of 

 the yeasts. 



33. Cell division is in most cases initiated by, or more 

 or less immediately preceded by, the division of the 

 nucleus. In coenocytes, on the contrary, this connection 

 between nuclear division and that of the coenocyte seems 

 to be lacking. Two types of nuclear division may be 

 distinguished, direct or amitotic and indirect or mitotic. 

 The latter process is generally known as mitosis or karyo- 

 kinesis. The direct division is comparatively rare and 

 appears to consist of a simple pinching in two of the nu- 

 cleus. By far the commonest method is that of mitosis. 

 This is a very complicated process and is essentially as 

 follows, ])eing subject, however, to many more or less 

 pronounced variations in different plants. If a centro- 

 some is present, which is apparently the case only in some 

 of the lower plants, it divides into two centrosomes which 

 move around outside the nucleus until thej- lie at oppo- 

 site sides in a line at right angles to the plane of division. 

 The nuclear reticulum now begins to resolve itself into a 

 fine tangled thread without cross connections, the chro- 



