358 CELL HEREDITY 



necessarily follow that they possess identical genotyjjes. This possibility 

 cannot be examined by breeding tests because the cells in question are 

 irre\ersil)lv determined as somatic cells. Such tests could be made with 

 organisms whose germ and soma are not irreversibly differentiated. It is 

 difficult to be sure that an organism meets this criterion because there is 

 alw avs the possibility that the sexual cells actually ari.se from progenitors 

 that have remained embryonic although they are surrounded by dif- 

 ferentiated neighbors. The same reservation must be held about studies 

 on the regeneration of differentiated parts of plants and animals. In uni- 

 cellular forms, such as the ciliate protozoa and certain algae like Acetah- 

 ularia, differentiation is reversible with no irreversible genetic change as 

 a consequence. In some animals it has been shown that different regions 

 of the cytoplasm of the egg are irreversibly determined as to their sub- 

 sequent fate before the nucleus divides. Thus something other than 

 nuclear differentiation must be responsible. In the classical experiments 

 that overthrew the theory of Weissmann, another German, Spemann, 

 showed that the nuclei of the developing egg of the salamander, at least 

 until the 16-cell stage, retain their ability to direct the formation of 

 a perfect embryo. 



The gonads of animals, themselves highly differentiated, provide an- 

 other test. By the application of hormones, sex reversal can be achieved 

 in, for example, the medaka fish. Genetically female cells thus form 

 sperm, and vice versa. But breeding tests show that such sex-reversed 

 sperm have a female genotype and the reversed eggs, a male genotype. 

 Despite the externally directed change in the course of differentiation, 

 the genetic constitution, insofar as it was testable, was unchanged. 

 Thus, the answer to our question about nuclear differentiation seemed 

 an emphatic "no, and we were confronted with a paradox: heritable 

 changes occur during development in cells whose nuclei are the same. 



We now know that this is not always true and also that the paradox is 

 anyhow resolvable. Transplantation of nuclei in the frog's egg shows 

 that irreversible nuclear changes do occur, not only during normal 

 development but also after residence in foreign cytoplasm. This suggests 

 that extranuclear factors may control somatic cell heredity through their 

 influence on nuclei. No chromosomal changes are apparent in these 

 cases, but in the dipteran insects, in certain tissues at certain times 

 during development, specific crossbands of the polytene chromosomes 

 enlarge into puffs (see Figure 11.7). In other types of cells at the same 

 time, different regions of the chromosome are puffed; in the same type of 

 cells at different times, different regions puff. The.se spatially differen- 

 tiated se(|uellae could be the morphological expression of the pattern of 

 gene states in time and space that brings about cell differentiation. 



