74 



informs the reader from the very outset that: "This book on 

 karyology marks the beginning of a new epoch, the transition 

 from an essentially descriptive to a largely deductive science. 

 ... It finds its parallel in the study of the fixed stars. The 

 average cytologist is primarily an observer and unaccustomed 

 to long chains of deductive reasoning. He will find the book 

 extremely difficult. . . . The chromosomes are not only aston- 

 ishingly similar in all organisms but are mainly responsible for 

 diversities of teeth and xylem. And their study immediately 

 discloses a whole set of new evolutionary principles which are 

 hidden from the macroscopic morphologist." 



Then follows the mapping out of the heavens of the new 

 cytology by Darlington. The instruments are in his hands. It is 

 he who with a consummate knowledge of experimental genetics 

 casts his eyes upon the nucleus and studies its stars and planets. 

 He marks their position, apportions their sizes, plots their 

 orbits, determines their magnitudes, and charts their constel- 

 lations. From the shades of color in the eye of Drosophila, from 

 the proportions of a squash, from the varying spines of a Jimson 

 weed he deduces when they must attract and when repel one 

 another, when there is partial or total eclipse, and when they 

 shall collide, and when they must fragment. 



There are still two kinds of nuclear divisions for Darlington 

 — as he calls them simple and double mitosis. Double mitosis is 

 really an abnormality of simple mitosis. It is called meiosis. 

 Chromosomes are two-parted and the preparation for the divi- 

 sion of the nucleus does not lead to the formation of a spireme. 

 The spireme concept is untenable on cytological and genetical 

 grounds. 



The two parts of the chromosome (chromatids) correspond 

 exactly part for part, chromomere for chromomere. The chro- 

 mosomes preserve their identity throughout all stages of mitosis 

 and during all succeeding division at succeeding mitoses. The 

 chromatids are cylinders and the chromomeres are arranged in a 

 single spiral. All chromosomes have one constriction unless it is 

 terminal. The spindle attachment always coincides with a con- 

 striction. The constriction is a point of weakness in the chromo- 

 some. Constrictions give diversity of form to metaphase chro- 

 mosomes and therefore character to individual chromosomes. 

 Chromosomes of one race because of their constant morphology 



