THE PHYSICAL BASIS OF HEREDITY. 185 



greatly increase this probability. The offspring resem- 

 bles both of its parents, and the paternal tendencies 



can be conveyed in the minute sperma- 

 The chromatin tozoan head alone, which is constituted 

 as the bearer of almost entirely of chromatin. Thescrup- 

 hereditary influ- , . . -^i l- l ■ u ..i- 



ulous exactitude with which, in both 

 ences. 



germ cells, the chromosomes are reduced 

 to one half the normal number preparatory to the union 

 of the pronuclei in fertilization, and the distribution of 



the paternal and maternal chromatin 



Indirect evi- equally to the resulting cells of cleav- 



dcncc 



age, lend added weight to the theory. It 



remained for the genius of Boveri by a brilliant experi- 

 ment to raise this hypothesis to the plane of almost ab- 

 solute certainty. The crucial test of the theory would 



be to remove the nucleus from one cell 

 Direct experi- ^^^ ^^ substitute it in the place of the 

 mental evidence. , , , -rr 1 1 j 



nucleus of another. If the nucleus and 



cytoplasm thus brought into union are so constituted 

 that they can exist together, then one of three things 

 will happen. Either the qualities of the cell from which 

 the nucleus was taken will develop, or those of the 

 cytoplasm, or those formed by the union of both nucleus 

 and cytoplasm. 



In this first case nuclear control will be demon- 

 strated ; in the second, cytoplasmic ; and in the third, 

 an interaction of both nucleus and cytoplasm will deter- 

 mine the activities of the cell. 



Such an experiment was first tried by Rauber in en- 

 deavouring to remove the nucleus of the fertilized frog's 

 egg, and to substitute for it the nucleus from a toad's 

 egg in the same stage of development. If the nucleus 

 contains the hereditary influences, then a toad must 

 develop from the union of frog's cytoplasm and toad 

 nucleus. The profound disturbances set up in the 



