INDIVIDUALITY OF CHROMOSOMES 301 



it is possible to follow the chromosomes from one cell to 

 another. In others there are certain size differences 

 among the chromosomes that appear unchanged from one 

 cell generation to the next, and hence suggest that here 

 also these bodies are self-perpetuating units. Also there 

 is a wealth of evidence in connection with breeding experi- 

 ments which practically amounts to definite proof that 

 chromosomes retain their individuahty. In other words, 

 four of the eight assumed chromosomes in every body- 

 cell are the direct descendants of those introduced by 

 the sperm, while the others are from the egg. Just as 

 individuals or cells are the lineal offspring through 

 thousands of generations of other individuals or cells, 

 so are chromosomes the descendants of pre-existing 

 chromosomes. 



Fertilization of Unicellular Animals. — Among 

 the unicellular animals where obviously there are no spe- 

 cial reproductive cells, there is nevertheless a fertilization 

 process that in its essential features resembles that of 

 higher animals. In Paramoecium, for example, two in- 

 dividuals come in contact and partially fuse together 

 for a few hours' time, and during the period undergo a 

 complicated series of nuclear changes. Without consider- 

 ing the process in detail it may be said that chromatin 

 material is exchanged and when the two cells separate 

 the nucleus of each is a fusion product as in the case of 

 the nucleus of the fertilized egg. Each individual thus 

 may be said to have fertilized the other. In several other 

 species there are small actively swimming individuals, 

 usually considered to be males, that permanently fuse 

 with and thus fertilize larger female individuals. 



It is generally believed that the introduction of new 

 chromatin into the cell results in a changed individual 

 that may be better fitted to perpetuate the race than the 

 previous type. But that the union of the two unicellular 

 organisms results in an increased capacity for cell division 

 is a debatable matter. In some individuals it appears to 



