SECT. I MORPHOLOGY 81 



may under some circumstances exhibit marked differences. On the 

 other hand, in multicellular organisms whole protoplasts are specialised 

 for functions with the performance of which their definite structure is 

 connected. 



2. Ontogeny of the Cell 



The Origin of the Living- Elements of Protoplasm. — All the 

 nuclei in an organism owe their origin to the nuclei of previous 

 generations. The spontaneous formation of a nucleus never takes 

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

 derived from pre-existing cytoplasm, and, so far as is yet known, 

 the chromatophores take their origin only from their own kind. 



Nuclear Division. — Except in a few well defined cases, nuclei repro- 

 duce themselves by mitotic or indirect division. This process, often 

 referred to as karyokinesis, is somewhat complicated, but seems 

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

 mother nucleus between the two new daughter nuclei. 



Indirect Nuclear Division (*2). — 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 the unstained 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 i). 



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 filaments 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 (i k). At this stage the nucleoli (nl) are dissolved and the 



