THE LINEAR DIFFERENTIATION OF THE CHROMOSOMES lOl 



the left arm of the Ilnd chromosome. These pair in characteristic 

 inverted loops; they have not yet been detected genetically, presumably 

 because no mutant allelomorphs of the genes concerned have been 

 discovered. There are also several small bilaterally symmetrical elements 

 which may represent similar inverted duplications in which the two 

 elements are actually contiguous. These phenomena provide evidence 

 that translocation and duplication has been involved in the evolution of 

 Drosophila melanogaster; we shall see later that a comparison of the 

 different species of Drosophila demonstrates the same thing. 



6. '' Lampbrush" Chromosomes^ 



During ovogenesis in some yolky eggs (sharks, amphibia, birds, etc.) 

 the diplotene chromosomes become enormously enlarged, being com- 

 parable in length to salivary gland chromosomes. The pachytene stage 

 may last a very long time, up to two years in Amphibia, and during 

 this period the chromosomes undergo a peculiar change whose exact 

 nature is not yet fully understood. In the early stages of the process, 

 the paired chromatids lie closely side by side and show a series of 

 swellings, which look much like other chromomeres. As the increase 

 in length continues, the chromomeres seem to give off filaments, 

 consisting of rows of granules, which lie transversely to the length of 

 the chromosome, and bend round to form loops. The sequences of 

 these loops seem to be characteristic of particular regions of the chromo- 

 some, just as are the sequences of bands in the salivary gland chromo- 

 somes, and presumably the growth in length of the whole chromosome 

 is a result of the uncoiling of the chromonemata to their full extent. 

 Eventually the loops and threads disappear from the sides of the 

 chromosomes, which contract again to a more usual size, and the 

 division proceeds. The formation and shedding of the side branches 

 releases into the nuclear sap, and thence probably into the cytoplasm, 

 substances formed under direct control of the chromosomes, and these 

 substances may perhaps play an important part in endowing the egg 

 with its developmental capacities. 



7. Genetic Analysis of the Process of Crossing-over 



The effect of temperature on crossing-over is shown only by gametes 

 which were in the prophase of meiosis when the abnormal temperature 

 was applied. 2 This is evidence that crossing-over occurs during meiotic 

 prophase;^ but the validity of this hypothesis rests mainly on the demon- 



^ Koltzoff 1938. Duryee, 1938. - Plough 1917 ^ Cf. Gowen 1929. 



