274 LILIAN V. MORGAN. 



types there are the usual three pairs of autosomes, but the sex- 

 chromosomes differ from the usual condition in that there is pres- 

 ent in both types a U- or blunt V-shaped chromosome in place of 

 a rod-shaped X-chromosome (Fig. 3, cf. a with b, c, d and e, f, g, 

 h} ; this body is undoubtedly the united X's derived from the 

 original mosaic. Besides the united X's from the mother, there is 

 in the yellow female the typical hooked Y-chromosome, the sex- 

 chromosome from the father (Fig. 3, e, f, g, h), and in the wild 

 type female the chromosome from the father is the typical rod- 

 shaped X (Fig. 3, b, c, d), whose presence gives her the wild type 

 body color of her father. Thus the cytological evidence brings 

 another confirmation of the chromosome theory of heredity. 



ORIGIN OF THE MOSAIC. 



The origin of the mosaic fly can be explained if at some division 

 in the spermatogenesis of the father (perhaps at the equation divi- 

 sion) the two halves of the X-chromosome failed to become com- 

 pletely detached, but remained fastened together at one of their 

 ends, producing the V-shaped chromosome or double chromosome 

 found in the germ cells of the female descendants. The sperm 

 containing this chromosome must have fertilized an egg containing 

 an X-chromosome with the differentials for forked and bar (this 

 is known to have been the constitution of half of the eggs of the 

 mother). An X-triploid fly was thus produced (cf. Fig. 2) ; the 

 triploid condition remained, however, in only the anterior part of 

 the fly, where the body color was wild type and the eyes were 

 slightly bar. These characters, as stated, are as expected in an 

 X-triploid, where two differentials for yellow are present with one 

 for wild type, and one differential for bar with two for wild type. 

 At an early stage the chromosome carrying forked and bar was 

 eliminated, leaving part of the fly (the abdomen) with only two 

 X-chromosomes, the attached yellow-bearing chromosomes (Fig. 

 2). The germ cells of the individual were derived from this part, 

 and at the reduction division of the eggs there must have been 

 extruded into the polar body (or remained in the egg) either the 

 two attached X-chromosomes or no chromosome. 



Dec. 1921. 



