stock." Seme investigators believe that man>^ of DeVries ' 

 Oenothera mutations arose by such a process. " 



Moreover, genes may not be immutable. As they are 

 complex bodies they are liable to undergo changes "due 

 either to the loss or addition of certain constituent atoms of 

 molecules, or to the rearrangement of some of these." 



The work of Morgan and his co-workers on eye-color in 

 Drosophila would seem to bear out this suggestion. "The 

 gene for the normal red-eye color may change so as to give 

 rise to blood, cherry, eosin, buff, tinged or white eyes. Genes 

 or allelomorphs that mutate in various directions give rise 

 to what are known as multiple allelomorphs; hypothetically 

 these may be explained as due to different changes, proba- 

 bly of a chemical nature, in a particular gene" (Conklin). 



The Individual Persistence of Chromosomes. -The 

 Chromosome Theory of Heredity takes for granted that in 

 all the successive nuclear divisions the individual chromo- 

 somes maintain their identity. 



Several investigators, including Boveri, Sutton and 

 Morgan, have discussed evidence that the chromosomes 

 that appear at the beginning of nuclear division are identi- 

 cal with the daughter-chromosomes of the preceding divi- 

 sion, and that each of the chromosomes plays its part in the 

 making of the organism. Frequently each may be re- 

 cognized by its shape and size. Boveri 's experiments with 

 the fertilized egg-cells of a sea -urchin where he removed 

 the nucleus of the egg-cell and a complete embryo was pro- 

 duced, showed that the set of chromosomes of one parent 

 alone were sufficient for the development of a normal indi- 

 vidual. This same investigator, however, found that 

 if one chromosome was wanting from either the paternal or 

 maternal set normal development would not take place. 



Although the chromosomes usually disappear after 

 division during the resting stage, in a number of cases they 

 have been traced through the resting stage until their emer- 

 g-ence at the next division. Wenrich has shown in Phry- 

 noteitix that the separate chromosomes by absorbing fluid 

 from the cytoplasm form vesicles and unite to form 

 the daughter nucleus. By the disappearance of the 

 partition walls a single nucleus vesicle is formed, but 

 under favorable conditions each chromosomal vesicle can be 

 seen to persist through the resting stage and at the next 

 mitosis the chromatic granules in each vesicle form a chro- 

 mosome like the one from which that vesicle arose. 



140 



