252 WILBUR WILLIS SWINGLE 
filaments. The side-by-side fusion or synapsis begins at the 
ends of the threads nearest the centrosome, and extends distally 
until fusion is complete throughout the length of the conjugants. 
Thus the amphitene is essentially a transition period in which 
the pairing of chromosomes in the stage of leptotene filaments is 
progressing. In the distal portion of the nucleus, where typical 
leptotene conditions persist, parasynapsis has not yet occurred. 
The evidence for this point of view is quite conclusive in Rana 
catesbeiana larvae : a) The leptotene threads are certainly nearer 
the somatic number than the haploid number; h) the thickened 
pachytene loops represent the haploid or reduced number; c) the 
thickness of the pachytene elements is just twice that of a single 
leptotene filament, d) and, perhaps most conclusive, it is not dif- 
ficult in studying amphitene nuclei, to trace the two unpaired ends 
of the leptotene threads from the distal pole into a single thick- 
ened pachytene thread at the proximal pole (figs. 11 and 12). 
Janssens ('05) has figured this stage clearly in his figures 20, 21, 
22, and 23, plate IV. Wilson ('12)' observed the same thing in 
Batracoseps material obtained from Janssens, and states that 
the conditions described are even clearer than Janssens figured 
them. Apparently analogous conditions are figured by Wenrich 
('16) (fig. 77) and designated by him as zygotene stages showing 
incomplete conjugation of chromosomes. 
The leptotene threads appear to coil or twist about each other 
corkscrew fashion so tightly that all trace of their double nature 
is lost and the resulting thickened thread appears single. (Fig. 12.) 
Many investigators of amphibian spermatogenesis have de- 
scribed other methods of synapsis for this group of vertebrates. 
However, the period assigned, which has usually been considered 
as identical with synapsis in urodeles and anurans, is in all prob- 
ability an artifact due to imperfect fixation of material, poor stain- 
ing, or both. This so-called synaptic period corresponds to what 
McClung has termed synizesis or the ''unilateral or central con- 
traction of the chromatin in the nucleus during the prophase of 
the first spermatocyte." Nuclear conditions are at this time 
extremely difficult to make out, to say nothing of interpreting 
correctly. The pachytene spireme is considered as evolving out 
of this contracted nuclear condition (King, '07, figs. 24 and 25) . 
