GENETICS 79 



ling center of the cell, and since everything con- 

 tained in it but the chromosomes is repeatedly 

 discharged into the cytoplasm and reformed in 

 daughter nuclei during cell division we may apply 

 the same interpretation to the chromosomes. The 

 additional fact that these bodies alone are univer- 

 sally maintained and perpetuated in a character- 

 istic state throughout the vicissitudes of normal 

 development and reproduction is further evidence 

 of their importance, and the many discoveries of 

 genetics which correlate the development of char- 

 acters with chromosome behavior are perhaps the 

 most convincing tribute of all to the type of control 

 which they exercise. 



Nevertheless even chromosomes are dependent. 

 The nucleus lies within a cytoplasmic envelope 

 which is necessary to its existence. It may be 

 removed by micro-dissection but cannot live in- 

 dependently so far as is known. Conklin ^ has suc- 

 ceeded by centrifuging eggs in producing daughter 

 cells of which one had a small nucleus, abundant 

 yolk, and a minute amount of cytoplasm. The 

 cytoplasm later increased in quantity; if none had 

 been present, it would be evident that the nucleus 

 alone might produce it, but this condition has not 

 yet been attained. Moreover cytoplasmic ma- 

 terials may pass into nuclei, so that the formation 

 of cytoplasm by a nucleus would not be evidence 



« Jn. Exy. ZooU Vol. XII, No. 1, 1912. 



