NEW TYPES AMONG ANIMALS 155 



merits show that when the eggs are cooled the number that hatches 

 is less than at higher temperatures. Thus the cool spells would 

 diminish the normal forms and relatively increase the abnormal. 

 All this is highly important, for we are working out certain far- 

 reaching biological principles which apply to man as well as to 

 the lower insects. 



In connection with Miss Hoge's experiments Plough, another 

 of Morgan's students, has carried on a still more conclusive 

 series. Using the same fruit-fly, drosophila, he has tested what 

 is known as "crossing-over." Microscopic study of the nuclei of 

 the reproductive cells shows that the factors which make up the 

 inheritance of any organism are carried by the minute threads 

 known as chromosomes. Ordinarily the corresponding chromo- 

 somes from the male and female nuclei fuse into one and then split 

 longitudinally in such a way that exactly half of each goes to 

 each of the cells of the new organism whose development is begin- 

 ning. Sometimes, however, the chromosomes do not unite in this 

 simple way, but wind about one another, or "cross over." Hence 

 when the splitting begins, the parts of the nucleus that go to the 

 two daughter cells are not exactly alike. This leads to a new 

 combination of characteristics in the offspring, so that decidedly 

 new types are formed. 



In his experiments Plough found that the amount of crossing- 

 over varies according to the temperature. This is illustrated 

 in Figure 25. The high parts of the curve represent the normal 

 conditions where the offspring resemble the parents according to 

 the Mendelian laws. The low parts mean that the percentage of 

 abnormal or new forms increases. The little hooks at the ends 

 of the curve are of little importance since they are based on 

 relatively few data, and might disappear if further experiments 

 were tried. At temperatures above and below those indicated in 

 the curve the flies perish. The meaning of the curve is evident. 

 When the mother flies are kept at low temperatures of 45° to 

 55 °F during the critical period of the development of the eggs 



