286 THEORETICAL BIOLOGY 



rule do not stop to consider that in it a problem is solved that 

 is mechanically impossible. 



I reproduce the diagrammatic pictures that Boveri, in 

 his great work, Das Problem der Befruchtung, gives of cell- 

 division, and this will enable every reader to get an idea of this 

 " miraculous " process. 



In Fig. I we see the resting ceU. In its protoplasm is 

 the little centrosome, which is placed above the cell-nucleus. 

 The cell-nucleus is free of protoplasm, and consists of the 

 scaffolding of chromatin, which is spread out like a sponge 

 in the nuclear sap. In Fig. 2 the centrosome has duplicated 

 itself, and at the same time the chromatin is set in motion. 

 In Fig. 3 the centrosomes have separated from one another, 

 while the mesh-work of the surrounding protoplasm arranges 

 itself in rays around them. Within the nucleus the chromatin 

 has clumped together into four strands, the so-called chromo- 

 somes. (The number of chromosomes is constant for each 

 species of animal. There may be from four to four hundred.) 

 In Fig. 4 the nuclear membrane has disappeared. The 

 nuclear sap is absorbed by the protoplasm, and the four 

 chromosomes lie free. In Fig. 5 the two centrosomes have 

 separated so far that they stand directly opposite one another. 

 At the same time, the radiation of the protoplasmic network 

 has increased. Four rays have meanwhile isolated them- 

 selves completely and on each side they become connected 

 with the chromosomes. In Fig. 6 the chromosomes now split 

 lengthwise in half, and the eight protoplasmic rays each draw 

 a half-chromosome towards the centrosome to which they 

 belong, the centrosome with its rays remaining firmly fixed 

 in the protoplasm (Figs. 7 and 8). In Fig. 9 the degeneration 

 of the division-apparatus begins, the chromosomes are once 

 more surrounded by nuclear sap, and the protoplasm has 

 divided itself into two halves. Fig. 10 shows the two resting 

 nuclei which represent the duplication of Fig. i. 



