HEREDITY IN SOMATIC CELLS 



357 



nu'iit l)ecause cells from which they ha\e been removed cannot multiply 

 and are at best moribund. But is the nucleus the primary site of the 

 changes that transform the egg into the multifarious cells that make up 

 the soma? It was proposed by the great German biologist, VVeissmann, 

 that mitoses gave rise to qualitatively different nuclei; he supposed that 

 this segregation of nuclear elements gave rise to the differences we see 

 among somatic cells. On what grounds was this provocative hypothesis 

 discarded? In many organisms, especially animals, the germ cells can be 

 distinguished early from the somatic cells. In the extreme and classical 

 example of the roundworm, Ascaris, the nuclei of the germ line receive 

 replications of all chromosome material of the fertilized egg, whereas 

 the somatic nuclei lack the ends of the original chromosomes eliminated 

 during cleavage (Figure 12.1). Apparently the germ line chromosomes 

 are aggregates, and many of the component chromosomes are unnecessary 

 for the development of the soma. Such supernumeraries are frequently 

 found. 



Nonetheless, no similar differentiation of chromosomes has been found 

 to distinguish the various types of somatic cells from one another. In 

 Ascaris the aggregate of supernumeraries enables the identification of the 

 germ line, but even in the soma, once the chromosome complement is es- 

 tablished, it remains constant. In other organisms where elimination of 

 chromosomes does not occur, the first, and each succeeding, mitosis in 

 the fertilized egg results in the production of what seem to be identical 

 sets of chromosomes. But, although they appear the same, it does not 



FIGURE 12.1. Chromosome elimination at the 

 second cleavage of the egg of Ascaris (after 

 Boveri, in: Ries and Gersch, 1953, Biologie der 

 Zelle, Leipzig, B. G. Taubner Verlagsgesellschaft). 



