THE REDUCTION OF THE CHROMOSOMES 



247 



If the conjugated chromosomes remain in contact at both ends (#4) a 

 complete ring results (F 4 ). In certain orthopterans the four chromatids 

 open out along the conjugation plane in some regions and along the plane 

 of splitting in other regions; this results in the curious compound rings 

 (Fig. 156) found in the cells of these insects. Finally, the chromatids 

 may open out from one end along the conjugation plane and from the 

 other end along the splitting plane (^5), the tetrad then assuming the 

 form of a ring composed of four parts (^ 5 ) . In all cases the tetrads usu- 

 ally condense into compact quadruple bodies by the time they take their 

 places on the spindle of the heterotypic mitosis. 







FIG. 97. Reduction with chromosome tetrads in Fasciola hepatica, according 

 to Schellenberg (1911). Explanation in text. 



The four chromatids composing the completed tetrad are in most cases 

 exactly similar in appearance, so that it is a matter of much difficulty to 

 determine along which plane they are separated at the first maturation 

 mitosis. According to the two theories of tetrad origin illustrated in 

 the foregoing diagram, however, the chromatids are supposed in almost 

 all cases to separate along the plane of conjugation at the first mitosis, 

 and this conclusion is supported by the behavior of those bivalent chromo- 

 somes which are not divided into tetrads of chromatids. 



A further interpretation of reduction involving chromosome tetrads 

 has been given by Schellenberg (1911) for the parasitic flatworm, Fasciola 

 hepatica (Fig. 97). The chromatin in the heterotypic prophase takes the 

 form of a long slender filament which splits longitudinally soon after 

 synizesis. This double thread then segments into the haploid number of 



