CHROMOSOME STRUCTURE 



179 



A somewhat different interpretation of the chromatic filament 

 has been suggested by Martens. In his investigations on Paris 

 quadrifoUa and Listera ovata, the chromosomes were observed 

 throughout the whole cycle of change, and the claim was made 

 that two morphologically distinct portions are concerned 

 in their structure, (1) a homogeneous achromatic matrix, which 

 is distinct from the nuclear sap, and (2) an embedded portion 

 or chromonematic element. During early prophase this chromone- 



. i a 3 4 5 , A 



Fig. 70. — Structural changes undergone by the chromosomes in somatic 

 mitosis in Paris quadrifoUa. (After Martens, from Sharp, Introduction to 

 Cytology.) 



matic portion has the structure of transverse or oblique curved 

 strands lying peripherally in the matrix, and many of these are 

 joined to form a zig-zag thread, but they do not constitute a con- 

 tinuous spiral throughout the length of the chromosome. During 

 the later stages of prophase they become continuous, and after 

 division of the chromosomes this chromonematic element, w^hich 

 at all times is accompanied by its achromatic matrix, still retains 

 its zigzag form until late in interphase, when it becomes broken 

 up into irregular strands. According to the view put forward by 

 this worker, the chromonema is a constant structural element of 

 the chromosome. 



