DF.VELOPMENT AND DIFFERENTIATION 



type co-operate only where they arc equally un£t, as in Uvularia, 

 or equally fit, as in Ascaris, where presumably the complete and the 

 diminished types of nucleus are, each of them, fitter for their own 

 types of tissue. But where the nuclei arc unequally fit, competition 

 at once eliminates the weaker and establishes genetic uniformity. 

 The Rcnner Effect shows also that one nucleus is not inherently 

 fitter or stronger than another; it is merely better fitted to the 

 cytoplasm in which it lies — A to that of the pollen, B to that of 

 the eggs. But both cytoplasms have been produced under the control 

 of the same AB nucleus. They differ merely in their developmental 

 history, in their cellular antecedents, in their chemical character as 

 parts of different tissues. 



Thus tissues, owing to their different relative positions in the 

 organisms, come to differ in the character of their cells, as shown 

 by the reactions of nuclei and cytoplasm. This is just the same 

 principle that we have already seen at work in Griineberg's lethal 

 in the rat, whose manifold effects on different organs can be traced 

 to a specific initial action on cartilage. The working of the nucleus 

 cartying this mutant gene breaks down when it is faced with a 

 specific job, that is when it lies in a specific cytoplasm. 



Transplantation experiments show us another aspect of the genetic 

 specificity of tissues. Hadorn transplanted into normal larvae organs 

 of the slowly developing "lethal-giant" Drosophila, at a time when 

 its imaginal discs were degenerating. He found that transplanted 

 testes degenerated, but the ovaries continued to develop and even 

 formed eggs. The cytoplasm thus determines whether and how the 

 gene will express itself. 



Further, Hadorn was able to select two lines throwing the 

 homozygous recessive lethal-giants. These giants died, in the one 

 early, and in the other late, in larval development. The lethal-giants 

 thrown by reciprocal crosses between these two lines took after the 

 mother line in the time of death. In other words the eggs differed 

 in cytoplasmic constituents which reacted differently with the lethal 

 gene and so determined the different times of death. These 

 constituents must themselves have been slowly produced gene 

 products, since the reciprocal difference was not continued in the 

 grandchildren. 



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