No. 632] CHIASM ATYPE AND . CROSSING OVER 



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only in the equatorial plane of the figure. The first of these cleavages 

 is here indicated by a dotted line in Fig. 1 A. (2) Moreover this plane 

 produces a longitudinal cleavage of the chromosome and hence is equa- 

 torial in each of the alternating chromosome segments (rings) that lie 

 exactly at the equator of the spindle of the figure (like that which 

 occurs at a gonial mitosis), Fig. 1 A and B. (3) Finally, since the two 

 spindles of the two maturation divisions that follow rapidly are per- 

 pendicular to each other, one may further add that each dyad will be 

 split during the maturation (maiotiques) division by two planes at 

 right angles to each other. At the first division, the equatorial division 

 plane is perpendicular to the axis of the heterotypic spindle and during 

 the second division the plane is in space, parallel to the original axis of 

 the same spindle. 



Janssens suggests the following considerations that 

 have an application to Mendel's laws. 



Neighboring segments pass easily into the same chromosomes when 

 the direction of the twist is constant. When a segment is long it may 

 be considered as carrying a longitudinal series of qualities, in con- 

 formity with the ideas held by Morgan. On the other hand the qualities 

 supposed to be carried by the chromosomal segments are distributed 

 amongst tlfe germ-cells as though they were carried by chromosomes 

 really independent confirming the law of disjunction of the characters 

 in the gametes (Mendel). 7 



Let us return to a further consideration of the dia- 

 grams that have been published to represent the methods 

 of crossing-over. In Janssens's scheme, Fig. 1 A, four 

 complete rings are represented and the division plane ap- 

 pears to cut through each node, although the important 

 details of how this is done are not shown in the figure, 

 but may perhaps be inferred from Fig. 1 C, where the 

 four vertical strands show what is supposed to have 

 taken place. Crossing-over is represented as having oc- 

 curred at four nodes. In Drosophila the genetic evi- 



. ^ Hence the phenomena of linka-o that 



regarded as of the utmost significance in < 

 bviously implied in Janssen's chiasma thee 

 that Janssens must intend to apply his 



