158 Harry Lewis Wieman. 



salts of heavy metals are used. For this reason I selected picro- 

 acetic acid as being least objectionable. 



Any process of fixation produces chemical changes in proto- 

 plasm which make the fixed material quite different from the liv- 

 ing. Whether or not a complete reversal in chemical reaction 

 occurs depends upon the amount of nucleic acid present. If this 

 is present in more than sufficient quantity to counter-balance 

 the tendency of the killing fluid to make the protoplasm positive, 

 the staining reaction would, of course, be the same as that in the 

 living material. This is undoubtedly true of the chromosomes, and 

 the evidence goes to show that it is probably true of these granules, 

 although the proportion of nucleic acid in the latter is much less 

 than in the former. 



In any event the presence of a cyclical process with alternating 

 phases of acidity and basicity is perfectly evident, since the same 

 killing agent is used at all times, and its specific action may be as- 

 sumed to be the same at all stages. The variable factor is the pro- 

 toplasm; the changing proportions of the basiphil anion, nucleic 

 acid, and the kation albumen radical, being responsible for the 

 different staining reactions obtained. 



Furthermore, these staining reactions are in keeping with what 

 one would expect from the theoretical side of yolk formation. 

 Yolk is regarded as an inert chemical substance, which implies 

 a low degree of chemical activity. Its reaction to the stain shows 

 it to be in an oxidized condition. The nutritive stream which is 

 the product of living cells shows an opposite reaction, indicating 

 chemical properties opposite to those of yolk. In the process of 

 forming yolk the basic staining granules of the food stream dis- 

 appear, and the acid staining yolk comes into existence as a pro- 

 duct resulting from the interaction of nucleus, cytoplasm and 

 food stream. 



The Nurse Cells 



The nurse cells are descendants of the primordial germ cells 

 whose reproductive function has been lost, and therefore they 

 are to be regarded as aborted eggs. The first result of the differen- 



