INDUCED CHROMOSOMAL ABERRATIONS IN ANIMALS 1197 



mutations with chromosome breakages. Muller and Altenburg (78), 

 Dobzhansky (30), Dobzhansky and Sturtevant (37), and Sivertzev- 

 Dobzhansky and Dobzhansky (124) discussed the applications of the 

 position-effect hypothesis in more detail. The association of definite 

 mutations with definite chromosome rearrangements (Sivertzev-Dobzhan- 

 sky and Dobzhansky, 124; Schultz and Dobzhansky, 116) is the strongest 

 evidence in favor of this hypothesis. 



The nature of position effect is not clear at the present. In fact, an 

 understanding of its nature presupposes a knowledge of the nature of the 

 gene. Mutatis mutandis, further studies on position effect are probably 

 the most promising mode of attack on the problem of the gene, perhaps 

 the most fundamental problem of genetics. Among many questions 

 which require investigation, one is, perhaps, outstanding: Are only the 

 genes immediately adjacent to the loci of breaks subject to position 

 effect, or may the function of genes lying at a certain, though small, dis- 

 tance from the breakage also be changed ? The alteration of the effects of 

 the genes lying in the duplicating fragments (see above) seems to argue 

 in favor of the latter possibility, but, unfortunately, it is exactly in these 

 cases that the proof of these alterations being due to position effect 

 (and not to disturbances in the genie balance) presents greatest difficulties. 

 If only the genes lying at the breakage points were changed, this would 

 argue in favor of the existence of intimate intergenic connections, perhaps 

 of the nature of chemical bonds of some sort between the molecules 

 representing genes. It would be then convenient to picture the gene 

 string in the chromosome as a chain of dissimilar molecules, somewhat 

 similar to the cellulose fibril, the links of the chain being connected by 

 definite bonds. If the position effect extends for a relatively considerable 

 distance from the breakage, the foregoing possibility is not excluded, 

 but then, perhaps the phenomenon is more easily accounted for by 

 supposing that the products elaborated by the genes react immediately 

 after being freed, the distances between the sources of these products 

 being of some consequence. 



MECHANISM OF MEIOSIS 



Up to a relatively recent date the process of meiosis was studied 

 primarily by determining cytologically the behavior of chromosomes in 

 gametogenesis of various species of animals and plants. Individuals 

 having normal chromosomes were usually selected for investigation. 

 This method of approach is essentially descriptive, observational, and 

 comparative. A wealth of valuable data was secured by this method: 

 the normal seriation of the different stages of meiosis became known; 

 the essential similarity of meiosis in widely dissimilar organisms was 

 revealed; a functional correlation between the cytological processes 

 and their genetic consequences established. The discovery of chromo- 



