268 



METAMORPHOSIS 



considerable dimensions and complicated form, and which still possess the 

 characters of a single cell ; but the great majority of cells divide after reaching 

 a certain size. In a typical case this takes place in the following manner. The 

 first alteration to be observed is in the nndeiis. In consequence of complex 

 internal processes, shown in Fig. 56, part of the chromatin first of all forms a much 

 twisted thread (2), which afterwards breaks up into a definite number of seg- 

 ments, the so-called chromosomes {^,4). In plants there are usually several such 

 chromosomes, which after taking on a U-shape, aggregate at the equator of the 

 nucleus. Each chromosome then splits longitudinally (>, 6), and the halves so 

 formed separate from each other in opposite directions (S, 9). Each group then 

 exhibits a fusing of the chromosomes into a network similar to that present in 

 the original nucleus {10-12). Thus there arise two daughter-nuclei separated 



Fig. 56. Mitosis and cell division in embryonic tissue. Somewliat diagrammatic. ;;, nucleolus; />, polar 

 plates ; w, nuclear wall ; cA, chromosomes ; s, spindle fibres (X 600). From the Bonn Textbook. 



from each other by an interval. The stages i-6 in the figure are collectively 

 spoken of as the prophase, 7-9 as the metaphase, 10-12 as the anaphase. In the 

 stage represented at Fig. 56,,/ we observe a system of fine threads stretching from 

 one ' pole ' of the nucleus to the other. They arise either from the ground sub- 

 stance of the nucleus (Zacharias, 1888) or, according to many authors, they 

 enter into the nucleus from the surrounding cytoplasm (Strasburger, 1888). 

 Certain of these spindle threads (' contractile threads '), by their contraction, 

 pull the chromosomes towards the poles (Fig. 57), the remainder persist as 

 connecting links between the two daughter-nuclei and form a basis for the 

 formation of the cell-wall. In the equatorial plane of the division figure (Fig. 

 56, JO, 11) there arise thickenings on the spindle threads, so that the individual 

 filaments approximate laterally at this point and forma protoplasmic wall {12), 

 the so-called cell or equatorial plate, which cuts the spindle medianly in two. 

 The cell-plate splits afterwards into two lamellae between which the cell-wall is 

 excreted (Strasburger, 1898). 



