274 WILBUR WILLIS SWINGLE 
parts of homologous chromosomes is well-nigh perfect, certainly 
more so than the later stages. In the first place, the side-by-side 
pairing of the two leptotene threads is accomplished by a process 
of twisting together, and not merely by a side-by-side union ; sec- 
ondly, the twisting of the homologous threads is so tight that all 
trace of their double nature is lost and the two elements appear 
as one. It is only later at the time of separation during the 
diplotene that the dual character of the pachytene thread again 
becomes apparent. It would be odd if during the process of 
separation of the tightly twisted threads 'crossing-over' did not 
sometimes occur (figs. 11, 12, 13, 36 to 38). 
Aside, however, from the ideal conditions presented by the 
amphitene stage for exchange of parts of chromosomes, the 
period figured by Janssens as furnishing actual cytological evi- 
dence of such exchange is not entirely satisfactory. The chief 
objection here is that study of the postspireme chromosomes of 
the maturation cycle in the larval bullfrog fails to support the 
contention that breaking and recombination of the chromatids 
occur at this particular period. The strepsinema stages of the 
bullfrog tadpole are very similar to those figured by Janssens, 
but the writer is of the opinion that crossing-over does not occur 
here; at any rate, no good evidence for it has been observed. 
The tetrads are of the non-cross-over type like those figured by 
Wenrich ('16) and Robertson ('14) for grasshoppers. 
Strepsinema stages of tetrad formation are perfectly definite 
and characteristic periods in the maturation cycle, and so far as 
myyinaterial is concerned, the chromatids appear to preserve their 
identity through this period (figs. 16, 40 to 63). 
There is certain genetical evidence indicative of chromosomal 
'crossing-over' during the early synaptic stages of the oocytes of 
Drosophila, such, for instance, as Plough's ('17) experiments. 
He found that environmental changes such as low or high tem- 
perature markedly increased the percentage of 'cross-overs' in 
the second chromosome of Drosophila melanogaster (ampelo- 
phila). The temperature apparently increased the amount of 
'crossing-over' at a definite stage of oogenesis, and Plough's evi- 
dence suggested strongly that the chromosomal exchange takes 
