19.s 



THE AMEBIC AN NATURALIST 



[Vol. L 



chromosomes, then, as above pointed ont, the separation 

 of factors in a group means chromosomal interchange. 



II. A Mechanism of Interchange already provided by 

 the Theory of Crossing-Over (Chiasmatype) 

 Janssen's " chiasmatype theory," based on cytological 

 observations of spermatogenesis in Batracoseps, described 

 just such a process of interchange between the homologous 

 chromosomes as Morgan's evidence from genetics required 

 (4) . A great bulk of evidence has accumulated to show that 

 during the period of synapsis, homologous chromosomes 

 come into contact, and in many cases chromosomes can be 

 seen to be twisted around each other during one stage or 

 another of synapsis. The essential point postulated by 

 the chiasmatype theory is that, as the paired chromosomes 

 draw apart again, they do not always untwist completely, 

 but may break at some points where they are crossed— 

 thus, in Fig. 1, the upper piece of the light-colored chro- 



L M 



mosome (L), which was on one side, loses its connection 

 with the lower part of L, that has crossed to the other side, 

 but becomes united instead with the lower piece of the 

 dark chromosome (M) which, on account of the crossing 

 of the strands, now follows it on the same side ; similarly, 

 of course, the upper part of M becomes united with the 



