DIV. i MORPHOLOGY 21 



E. Origin of the Elements of the Protoplast ( 4 ) 



All the living elements of the protoplast, the cytoplasm, the nucleus, 

 and the chromatophores, are never newly formed but always arise from 

 the corresponding elements of previous generations. They increase 

 in mass by a process of growth, BUT THEY INCREASE IN NUMBER, LIKE 



THE PROTOPLAST AS A WHOLE, ONLY BY DIVISION OF THEIR KIND. In this 



way the properties of the living constituents of a germ cell are trans- 

 mitted to all the cells of an organism and ultimately to its reproductive 

 cells, the uninterrupted continuity of the life being maintained. The 

 division of the protoplast is usually initiated by the division of the 

 nucleus. In the case of uninucleate cells this intimate association of 

 nuclear- and cell-division is necessary in order to ensure that each 

 daughter cell has a nucleus. In the multinucleate cells (e.g. of Algae 

 and Fungi) this is not essential, since each daughter protoplast 

 would obtain the requisite nuclei, and as a matter of fact cell division 

 in such cases is often independent of nuclear division. 



It sometimes happens that the protoplast of a cell, without 

 dividing, abandons its old cell wall. This process, which is called 

 REJUVENATION of the cell, has nothing to do with cell division. 



The rounding off of the protoplast in a cell of the green alga Oedogonium, and 

 its emergence from an opening in the old cell wall as a naked swarm-spore, is an 

 example of rejuvenation. Another is afforded by the protoplasts of the spores of 

 mosses or ferns and of the pollen-grains of seed-plants surrounding themselves with 

 a new cell wall within the old membrane, which then perishes. 



1. Typical Division of the Protoplast, (a) Nuclear Division. 

 Except in a few cases, nuclei reproduce themselves by MITOTIC or 

 INDIRECT DIVISION. This process, often referred to as KARYOKINESIS, 

 is somewhat complicated. 



Indirect Nuclear Division ( 20 ). 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. 13) as they occur in a vegetative cell such as 

 those which compose the growing point. 



The fine network of the resting nucleus (Fig. 13, 1 n) becomes 

 drawn together at definite points and separated into a number of 

 bodies (Ffg. 13, 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, 4), and stain more 

 deeply. The filaments are called CHROMOSOMES. Each chromosome 

 undergoes a longitudinal split which continues to become more 

 marked (5). The chromosomes, which become shorter, thicker, and 

 smooth (6), are moved into the plane of division where they constitute 

 the nuclear or equatorial plate (7 kp\ a stellate figure (aster) which 



