SECT, i MORPHOLOGY 81 



generations. The spontaneous formation of a nucleus never takes 

 place. In the same manner, the cytoplasm of every organism is 

 derived from the cytoplasm of the germ cell, and, so far as is yet 

 known, the chromatophores take their origin only from their own 

 kind. 



Nuclear Division. Except in a few limited cases, nuclei reproduce 

 themselves by MITOTIC or INDIRECT DIVISION. This process, often 

 referred to as KARYOKINESIS, is somewhat complicated, but seems 

 necessary in order to effect an equal division of the substance of the 

 mother nucleus between the two new daughter nuclei. 



Indirect Nuclear Division ( 82 ). In its principal features the pro- 

 cess is similar in the more highly organised plants and in animals. 

 Its stages are represented in a somewhat diagrammatic manner in the 

 following figure (Fig. 86), as they occur in a vegetative cell such 

 as those which compose the growing point. 



The fine network of the resting nucleus (Fig. 86, 1 n) becomes 

 drawn together at definite points and separated into a number of 

 bodies (Fig. 86, 2 ch), the outline of which is at first irregular. Their 

 form soon becomes filamentous, and the filaments become denser and 

 at the same time shorter and thicker (3), and stain more deeply. 

 The stainable substance of the filament, which is called chromatin, 

 becomes arranged in more or less regular transverse discs united by 

 linin (3). The filaments themselves are called CHROMOSOMES (Fig. 86, 

 3, 4). The chromosomes are moved into the plane of division where 

 they constitute the nuclear or equatorial plate (5 kp, 6, 7). Each 

 chromosome has meanwhile undergone a longitudinal split which 

 continues to become more marked (5, 6, 7). The two halves of 

 each chromosome thus separated move away from one another in 

 opposite directions, and take part in the formation of the daughter 

 nuclei (9 t). 



Other changes serve to direct the process thus briefly described. 

 While the nuclear network is separating into the individual chromo- 

 somes, cytoplasmic filaments become applied to the nuclear membrane, 

 surrounding it with a fibrous layer. This layer becomes raised up 

 from the nuclear membrane at two opposite points (3 k) and forms the 

 polar caps. These are filled with a homogeneous substance in which 

 fine filaments appear later. The latter converge at the poles, without, 

 however, coming into contact ; they constitute two pointed bundles, 

 since they diverge from one another as they pass from the polar 

 regions (4 &). At this stage the nucleoli (nl) are dissolved and the 

 nuclear membrane disappears. The fibres proceeding from the polar 

 caps can thus become prolonged into the nuclear cavity (4, 5). Here 

 they either become attached to the chromosomes, or^ filaments from 

 the two poles may come into contact and extend continuously from 

 the one pole to the other. In this way the nuclear spindle is formed 

 (5, 6, 7). The fibres of the spindle attached to the chromosomes may 



