254 GERM-CELL CYCLE IN ANIMALS 
chromosomes. The chromosomes may unite side by 
side in parasynapsis or end to end in telosynapsis. 
Apparently parasynapsis is the rule, although telosyn- 
apsis probably occurs in certain species. The results 
are the same in either case. 
The next question to be considered is whether the 
chromosomes which emerge from the pachytene stage 
are the same as those that enter it as leptotene 
filaments, or whether there is a complete fusion 
into zygosomes or mixochromosomes. It seems 
probable that at least a partial fusion occurs 
and that the composition of the chromosomes is 
changed more or less during synapsis. We know 
for certain that the peculiar X-chromosomes which 
have been found in many species of animals become 
paired in synapsis and later separate in a true 
reduction division, and we also have evidence 
which furnishes a mechanical means of effecting 
a change in the chromosomes during the synaptene 
stage. This evidence has led to the formulation of 
the chiasmatype theory (Janssens, 1909). Accord- 
ing to this theory the chromosomes which pair in 
synapsis may twist around each other more or less 
(Fig. 63), and cross connections are visible. When 
the paired chromosomes later split apart they rep- 
resent combinations different from those present 
before synapsis, because of these cross connections. 
The results of experimental breeding seem to necessi- 
tate some such relation as this during synapsis, and 
the chiasmatype theory has been used to explain 
certain results of hybridization that have not been 
