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 

 i^ito 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 



