162 



mosome rearrangements of known 

 genetic character, we can give mor- 

 phological positions to gene loci and 

 construct chromosome maps with far 

 greater exactness than has been hereto- 

 fore possible. 



(2) In old larvae, homologous chro- 

 mosomes undergo a process of somatic 

 synapsis. This union is more than a 

 simple apposition, for the elements pair 

 up line for line in the most exact way 

 and form one apparent structure. If 

 one of the homologues carries an in- 

 verted section we get typical inversion 

 figures, such as we would expect in 

 meiosis. If one of the homologues is 



PAINTER 



deficient, at some point, the two mates 

 unite except at the point of deficiency 

 where the normal element usually 

 buckles. Thus we can readily deter- 

 mine exactly how much of the one 

 chromosome is missing. It is probable 

 that the force which causes homo- 

 logues to unite in salivary glands is the 

 same that operates in meiosis, and 

 while, so far as is known, these special- 

 ized chromosomes never divide, we 

 can at least study how aberrant chro- 

 mosomes unite at synapsis, a fact 

 which should prove of great value to 

 geneticists. 



(3) In salivary glands the two arms 



of the V-shaped autosomes appear as 

 independent elements with no obvious 

 connection between them. As a result, 

 after somatic synapsis, we find six ele- 

 ments in the nucleus, not the haploid 

 number. 



(4) The inert region of the X-chro- 

 mosome does not appear as an organic 

 part of this element, nor does it show 

 in any other as yet recognized form in 

 the nucleus. Likewise, the only part of 

 the Y-chromosome which has been 

 identified is a short piece which, mor- 

 phologically, is homologous to part of 

 the right-hand end segment of the X. 

 This part of the X (see figure) carries 

 the normal allelomorph of bobbed. 

 Either the inert material of both the 

 X and Y has been eliminated during 

 ontogeny, by diminution or some sim- 

 ilar process, or this material exists in 

 the salivary nuclei in some unrecog- 

 nized form not visibly connected with 

 the chromosomes. The inert area com- 

 prises about Ys of the volume of the 

 oogonial metaphase chromosomes. 



The accompanying figure is a draw- 

 ing of the X-chromosome made by 

 uniting camera lucida sketches of vari- 

 ous regions. Fine details are omitted. 

 Above the figure, a crossover map 

 having the same length as the X is 

 shown. The symbols of gene loci, 

 which have been located, are given 

 together with lines showing their ap- 

 proximate morphological positions. 

 The points of breakage are indicated 

 on the X, with the name of the break 

 given below. Thus, deletion 14 (at the 

 left) broke the X between the loci of 

 scute and broad. The morphological 

 point of breakage is shown on the 

 drawing, and, of course, scute must lie 

 to the left of the break and broad to the 

 right. In a similar way the position of 

 other gene loci has been determined. 

 Geneticists will be interested to note 

 the morphological (and genetic) limits 

 of ClB and delta 49 inversions as 

 shown by the figure, and in the close 

 correspondence between the cytologi- 

 cal and crossover maps. 



