198 PROCEEDINGS OF THE ACADEMY OF [Feb., 



spermatocyte stage), and two such univalent heterochromosomes 

 might or might not later conjugate in a spermatogonium to form an 

 unpaired heterochromosome. On such a premise paired heterochromo- 

 somes and chromosomes within the spermatogonia would be an earlier 

 condition than unpaired ones, and unpaired heterochromosomes could 

 be formed in two ways. 



The conclusions of the preceding paragraph are put forward merely 

 as tentative suggestions, and in no sense as final conclusions; the phe- 

 nomena are too complex as yet for any thorough analysis and interpre- 

 tation. But amongst all this complexity a certain agreement in the 

 phenomena becomes evident, and this it is our business to discover. I 

 still see no reason, despite the criticisms of McClung, to modify my 

 original standpoint (19016), that there is a transmutation in chromo- 

 somal numbers just as in any other parts of the organization, and that 

 the heterochromosomes are chromosomes on the way to disappearance ; 

 following Paulmier's (1899) earlier contention that they are degenerated 

 chromosomes. McClung (1900, 1902) mges that they are frequently 

 larger than other chromosomes and show just as many signs of active 

 metabolism. But neither Paulmier nor I regarded them as dead 

 structures ; and I pointed out that they seem to have a different meta- 

 boKc energy from the ordinary chromosomes, because in some species 

 of Hemiptera they are regularly attached to the true nucleolus, which 

 condition the other chromosomes do not share, and have a different 

 position within the nucleus (almost always against its membrane). 

 There can well be no question that they are metabolically different, else 

 they would not behave so differently, with a peculiar autonomy. 

 McClung has described them only for Orthoptera, where they are fre- 

 quently the largest chromosomes. But the paired heterochromosomes 

 of the Hemiptera are usually the smallest of all, sometimes very minute 

 granules (as in Perihalus, Coenus, Trichopepla, Corizus, Coriscus, Pri- 

 onidus) ; and when there are several pairs within a cell, as, e.g., Acholla, 

 all of them are smaller than the other chromosomes. So I considered 

 them degenerate in the sense that they no longer carry on exactly the 

 same activities as the ordinary chromosomes, from which they must 

 be derived, but have taken on other energies and have in most of the 

 described cases become smaller. The excessively minute hetero- 

 chromosomes would then be the last perceptible stage in their history; 

 for surely there is no reason to consider this the first stage — to consider 

 them as orginating as buds from larger ordinary chromosomes. Un- 

 paired heterochromosomes do not conjugate during the growth period, 

 for the reason of the absence of a mate with which to unite; and in 



