CROSSING OVER AND CHROMOSOMES 111 



the linear order of the material of the chromosomes cannot 

 be overestimated. As a further example Wenrich gives 

 identical stages of the same chromosomes (Fig. 49), each 

 of the figures is from a different individual. The identity 

 in size and in location of the principal beads in the series 

 is obvious. 



Robertson has also brought forward a case of an 

 unequal pair of chromosomes and interpreted the facts as 

 opposed to the crossing-over hypothesis. He found two 

 cases in a grasshopper of the genus Tettigidea in which 

 there was a very unequal pair of chromosomes. The 

 shorter piece conjugated consistently with only one part 

 of the longer chromosome, as shown in the next figure 



mMMMSC 



a 



b 



d 



Fig. 50 — Conjugation of an unequal pair of chromosomes and their subsequent separation. 



(After Robertson.) 



(Fig. 50, a,h). At the first maturation division the two 

 chromosomes separated, as shown in (c, d, e). It would be 

 difficult to find a more excellent illustration of the per- 

 sistence of the individuality of the chromosomes after con- 

 jugation, and the case falls equally in line with the view 

 that conjugation takes place only between those parts of 

 the chromosome that are alike, i.e., composed of the same 

 series of genes. How, then, could this case, so admirably 

 suited to support the chromosome theory be turned against 

 the chiasma theory ? Only, I think, through a misconception 

 of the essence of the theory. Robertson says: *'In both 

 types of unequal tetrads we have very strong evidence that 

 the homologous chromosomes, on entering the side-to-side 

 pairing process of synapsis, remain as distinct individ- 

 uals, retain their identity throughout the period, and come 



