152 GENERAL BIOLOGY 



some interaction between the cytoplasm of two fusing 

 individuals, even if the nuclei remain individually 

 distinct. 



Nuclear Conjugation (Karyogamy) . In the great 

 majority of cases, however, both in plants and 

 animals, the essential feature of zygosis appears to 

 be the fusion of the nuclei of the two conjugating 

 gametes. Just as it seems necessary for the cyto- 

 plasm to be in a peculiar physical (and doubtless 

 also chemical) condition before plastogamy can take 

 place, so also of the nucleus. But whereas the pre- 

 liminary nuclear changes are perhaps more compli- 

 cated and subtle than those of the cytoplasm, they 

 are easier to observe. 



In all the Metazoa and Metaphyta, specialization 

 has progressed to the point of differentiation between 

 somatic cells and germ-cells. The latter cells share 

 with the former a common ancestry ; that is to say, 

 in any one individual they must have all descended 

 from a single zygote. There is reason for believing, 

 however, that the germ-plasm is differentiated from 

 the soma-plasm at the beginning of individual 

 development. It is therefore not a matter of indif- 

 ference which cells become gametes. But since the 

 individual itself must come to a point of sexual 

 maturity before its gametes are able to continue 

 the existence of the race by conjugating with other 

 gametes, sexual maturity of the individual is coinci- 

 dent with the maturity of its gametes. In other 

 words, in spite of the fact that the germ-plasm is a 



