180 THE CELL 



The main feature of the process consists in this, that the various 

 chemical substances (vide p. 40), which are present in the resting 

 nucleus, undergo a definite change of position, and the nuclear 

 membrane being dissolved, enter into closer union with the proto- 

 plasmic substance. During this process the constant arrangement 

 of the nuclein becomes especially apparent; and, indeed, the 

 changes, which occur in this substance, have been most carefully 

 and successfully observed, whereas we are still very much in the 

 dark concerning what takes place in the remaining nuclear sub- 

 stances. 



The whole mass of nuclein in the nucleus becomes transformed 

 during division into fine thread-like segments, the number of 

 which remains constant for each species of animal. These seg- 

 ments are generally curved, and vary in form and size according 

 to the individual species of plant or animal ; they may appear as 

 loops, hooks, or rodlets, or if they are very small, as granules. 

 Waldeyer (VI. 76) proposed the common name of chromosomes for 

 all these various forms of nuclein segments. As a rule I shall 

 employ the more convenient name of nuclear segments, which 

 applies equally to them all, whilst, at the same time, the expres- 

 sion indicates the most important part of the process of indirect 

 division, which consists chiefly in this, tha( the nuclein breaks up 

 into segments] Similarly the term nuclear segmentation appears 

 to me to be preferable to the longer and less significant expression 

 of indirect nuclear division, or the terms mitosis and karyokinesis, 

 which are incomprehensible to the uninitiated. 



During the course of division each nuclear segment divides 

 longitudinally into two daughter segments, which for a time lie 

 parallel to one another, and are closely connected. Next, these 

 daughter segments separate into two groups, dividing themselves 

 equally between the two daughter-cells, where they form the 

 foundation of the vesicular daughter nuclei. 



The following phenomena are also characteristic of the process 

 of nuclear segmentation : (1) the appearance of the two so-called 

 pole corpuscles (centrosomes), which function as central points, 

 around which all the cell constituents arrange themselves ; (2) the 

 formation of the so-called 'nuclear spindle ; and (3) the develop- 

 ment of the protojdasmic radiation figures around the centrosomes. 



As regards the two centrosomes, they make their appearance 

 in the vesicular nucleus at an early stage, before the membrane 

 has been dissolved, being situated in that portion of the proto- 



