PREDETERMINATION OF SEX 301 



new threads are made up of parts of each of the two orig-inal 

 threads. Under such circumstances a single maturation divi- 

 sion would often produce a dyad in which the two threads are 

 genetically unlike. Janssens assumes that this is repugnant to 

 the scheme of reduction, whose purpose is to give rise to a 

 gamete with a single set of units (gens). This solution of the 

 problem rests on the supposed necessity of pure gametes, and 

 this in turn could be more directly accomplished, it would seem, 

 if the conjugating chromosomes separated without interchanging 

 parts, as they do, in fact, in the male of Drosophila. Granting, 

 however, that 'crossing over' does occur as a necessity of the 

 physical conditions prevailing at this time, Janssens' hypothesis 

 might appear to furnish a solution, if at the same time the ne- 

 cessity for pure gametes could be shown essential to development. 

 Obviously, however, the act of conjugation is a capital arrange- 

 ment to give the zygote a heterozygous make-up, and why a 

 quadruple set of gens instead of a double one would be a disad- 

 vantage is not self-evident. 



A TETRAPLOID CYST IN THE BEARBERRY APHID 



In a male of the bearberry aphid one cyst was found in which 

 all the' cells have the double number of chromosomes including 

 the sex chromosomes. The cyst is in the first spermatocyte 

 stage. Since the other cells in the same testis and in the testis 

 of the other side are normal the tetraploid condition must have 

 arisen from a spermatogonial cell whose chromosomes but not 

 the cytoplasm divided. The other possibility, namely, that the 

 four autosomes failed to conjugate, would not account for the 

 presence of two X chromosomes, nor explain the doubling in all 

 the cells of the cyst, because conjugation occurs long after the 

 cells of a cyst have become separated. The former interpreta- 

 tion is therefore to be preferred. The cells in this cyst, of which 

 a few are shown in. plate 2, figures 21-30, are completing, or else 

 have completed, the first spermatocyte division. Taking the 

 figures in order, we see in f gure 21 four autosomes at each pole — 

 only three show in the larger cell — and two X's extending from 



