THE MECHANISM OF DEVELOPMENT 



H7 



has been going on. The centrosome has divided and the two 

 new bodies derived from it have separated and migrated to 

 opposite sides of the nucleus, each surrounded by its radiating 

 lines, in which condition they are known as "asters" (stars). 



During this migration the asters are generally (not always) 

 visibly connected by lines, but in either case by the time they 

 have reached opposite sides of the nucleus they will be seen to 

 lie at opposite ends of a spindle-shaped body (the amphiaster) 

 consisting of lines, among which lie the chromosomes. 



Matters are now ready for the final and significant acts of 

 cell division. The chromosomes arrange themselves end to end 

 along the equator of the spindle, and at right angles to its axis ; 

 whereupon each chromosome splits lengthwise, one group of 

 halves migrating to one aster (centrosome), the other to the 

 other, where each clusters about its own center, forming a new 

 nucleus with its centrosome. The cell wall now becomes con- 

 stricted, dividing the cytoplasm approximately equally (some- 

 times very unequally) between the two new cells, and the division 

 is complete. The resting stage ensues, during which preparation 

 is made for another division ; indeed, the centrosome occasionally 

 divides, in anticipation of the next division, even before all the 

 details of the first division are complete. For a graphic outline 

 of the complete process of mitosis see Figs. 20 and 21. 



This, in general, is the process of cell division which, with 

 more or less variation, attends all growth. The significant facts 

 brought to light in this complicated process are: (i) that the 

 number of chromosomes is constant for all individuals within 

 the species ; (2) that for all forms arising by sexual reproduc- 

 tion the number is even ; (3) that however its details may vary, 

 cell division consists essentially in a splitting of the chromo- 

 somes, by which each daughter cell secures (apparently) an 

 exact equivalent of what is received by the other daughter cell 

 of the same division. Cell division is therefore not a lump 

 division of the cell mass, but it is meristic, insuring a strictly 

 qualitative division in which one half of each chromosome 

 descends to either daughter cell. 1 



1 These same facts have added significance when considered in connection 

 with the germ cells, reproduction, and the problems of heredity. 



