82 LECTUEES ON BIOLOGY 



chromosomes is reduced by one-half. We shall deal with this 

 fact later on. 



If we observe highly magnified chromosomes we may some- 

 times see a fine crevice extending over their entire length, 

 but in most cases they appear as yet uniform. Gradually 

 the nuclear membranes become indistinct, and finally disappear 

 altogether. It now seems as if the chromosomes lay perfectly 

 free in the cell-protoplasm. Further observation will, however, 

 show that to each chromosome there have become attached 

 some of the ' spindle-filaments ' radiating from the central 

 bodies. Along these rays the chromosomes wander towards 

 the centre, and finally range themselves in a plane perpen- 

 dicular to the longitudinal axis of the spindle, the so-called ' equa- 

 torial plane. ' This process finishes the first stage of mitosis, the 

 prophase, and now commences the metaphase, the climax of 

 indirect nuclear division (fig. 23, 2 and 3 ). 



It is a strange and surprising spectacle which unrolls itself 

 before our eyes. Suddenly, and with absolute one might almost 

 say mathematical exactitude all the chromosomes split along 

 their length, like logs of wood. This division proceeds with 

 such precision that it seems unquestionably a process of vital 

 importance. The result of it is that the chromatin of the mother- 

 nucleus is distributed among the two newly formed daughter- 

 nuclei on the principles of strictest justice. There is no other 

 substance in the whole cell the distribution of which is watched 

 over with such care, and it is therefore not surprising that a 

 special function is ascribed to it. Quite early it was dis- 

 covered that chromatin was the ' bearer of the hereditary 

 qualities,' an hypothesis which, as we shall see later on, was 

 afterwards confirmed by many investigators. 



Events now rapidly tend to the end. Just as in a dolls' 

 theatre the operator regulates at will the movements of his 

 marionettes by means of numerous strings held in his hands, so the 

 central bodies appear here to control each step of the chromo- 

 somes. One gains the impression that the spindle-fibres adher- 

 ing to the chromosomes suddenly shrink, as if impelled by an 

 invisible force, thus forcing the longitudinal sections of the 

 chromosomes nearer to the corresponding central bodies. In this 

 way we obtain the stage of the two daughter-plates (fig. 23, 4). 



