14 THE ORIGIN OF GYNANDROMORPHS. 



to females that result. Boveri thought, from the evidence obtained, 

 that the loss of one chromosome at this time is a constant phenomenon. 

 If so, it differs in this regard from the rare occurrence of elimination 

 in Drosophila. 



In the group of aphids and phylloxerans a process occurs that has at 

 least a certain analogy to elimination. When the male-producing egg, 

 which is smaller (in the latter group) than the female-producing 

 egg, throws off its single polar body, one sex chromosome is eliminated 

 from the egg, although the autosomes divide equationally at this 

 time. This elimination is not due to loss of a daughter chromosome, 

 because it is preceded by a sort of synaptic union and disjunction of the 

 chromosome in question. Here the lagging of one whole chromosome 

 in the middle part of the spindle, and its failure to reach the outer 

 pole in time to become incorporated in the nucleus of the polar body, 

 furnishes a certain resemblance, at least, to the elimination process. 



In one species, P. fallax, there are four sex chromosomes, two of 

 which are eliminated from the male-producing egg, as described above. 

 There remain, then, two sex chromosomes in the male. When the 

 Sperms are produced these two do not act as mates when the other 

 chromosomes (autosomes) pair and segregate, but both pass together 

 to one pole. The daughter cells that get them become the functional 

 female-producing spermatozoa; the other cell that lacks them de- 

 generates. Here, then, although two sex chromosomes are present, 

 they both pass to one pole. This behavior is quite unlike the results 

 produced by chromosomal elimination. 



In one of the aphids Morgan found a cyst in which, owing apparently 

 to the failure of the autosomes to pair before segregation, an irregular 

 distribution of the chromosomes took place, including an erratic dis- 

 tribution, somewhat imperfect, it is true, of the sex chromosomes 

 also. This unusual and irregular occurrence might lead to complica- 

 tion in the distribution of the sex chromosomes in the next generation, 

 if such sperm were to become functional, and furnish a parallel case 

 to the phenomenon of primary non-disjunction that Bridges has 

 described in Drosophila. 



In Drosophila there takes place on rare occasions an erratic distribu- 

 tion of the sex chromosomes, either in the male or in the female, that 

 has been called primary non-disj unction. Occasionally, both sex chromo- 

 somes are eliminated in the polar body, leaving in the egg the haploid 

 number of chromosomes, but not a sex chromosome. If such an egg is 

 fertilized by a female-producing sperm containing one X chromosome, 

 an XO male results. The male, lacking the characteristic Y chromo- 

 some of the normal male, nevertheless resembles a normal male in all 

 respects, except that he is sterile. Conversely, in other cases, both X 

 chromosomes may remain in the egg. Such an egg does not develop 

 if it is fertilized by a female-producing sperm giving it three X's, but 



