THE GROWTH OF THE CELL 



269 



If now, as is often observable, the nuclear spindle broadens, so that it 

 occupies the whole diameter of the cell, the new cell-wall placed at right angles 

 to the old membrane divides the cell at once into two halves. A ' simultaneous ' 

 cell-wall formation of this sort occurs especially in narrow cells. It is not con- 

 fined to such however, but occurs also in broad cells, when the nuclear spindle 

 extends across the whole cavity of the cell (Fig. 56, 12). 



In addition to the simultaneous type we also not infre- ^ 2 



quently meet with a gradual or ' succedaneous ' forma- 

 tion of the cell-wall. The new wall, as in Spirogyra, 

 may extend inwards from the mother cell-wall in the 

 form of a plate or ridge, with a central aperture which 

 gradually becomes contracted (Fig. 58), or the forma- 

 tion of the wall may commence at a definite spot on 

 the wall of the mother cell and slowly grow across 

 it (Fig. 59). In this latter case the nuclear spindle 

 moves forward as the new cell-wall gradually crosses 

 the mother-cell. After the wall has been formed, the 

 remainder of the nuclear spindle left over in the two 

 daughter-cells disappears. In a few cases, such as 

 that of Oedogonium, the complete wall is formed 

 within the cell and unites later with the older cell-wall 

 (compare Hirn, 1900) ; as a rule, however, the wall is 

 built out from the spot where it originates. 



The division of the mother-cell takes place in a quite definite manner, and 

 the laws governing cell division were first, at least in part, elucidated by Hof- 

 MEiSTER (1867) and Sachs (1878-9), but the sequences were worked out more 

 accurately by Errera (1886) and Berthold (1886). Both authors found that 



Fig. 57. Diagram to illustrate 

 the separation of the chromosomes. 

 ^, spindle threads; a aiiJ 6, the 

 two longitudinal halves of the 

 chromosome. (From the Bonn 

 Textbook.) 



w rJi cJb 



Figf. 58. Division of a cell oi Spirogyra. w, daughter- 

 nucleus; w, partition wall ; ch, chloroplast (x 230). 

 From the Bonn Textbook. 



Fig;. 5g. Cells of Epipactis palnstris in three 

 successive stages of division. After Treub, from 

 the Bonn Textbook (x 365). 



in the majority of cases the new walls were placed across the old ones exactly 

 as would be the case with a soap film. The laws of arrangement of liquid lamellae 

 have been closely studied and we know that these lamellae always show what 

 are termed ' minimum surfaces '. If a soap film be stretched diagonally across 

 a cube (Fig. 60, /) it will shift its position until it has attained a minimum area, 

 that is to say, in this case, until it has divided the cube into two parallelopids 

 (2). If we extend a partition in the cube quite close to one of the walls and 

 parallel to it, it shifts itself and bends until it has cut off a segment of the cube 

 (Fig. 60,^). Similarly in the plant-cell partition walls appear, sometimes straight, 

 sometimes curved, but to study the details of these would take us too far. As we 

 have already said, they exhibit in the majority of cases minimum surfaces, save 



